Even in D&D, only the absolute strictest reading of alignment would make that true.
Yes.
Alignment is strict ... that may be one of reasons why WotC is distancing from it lately.

There is very important difference between being good person, and having good alignment ...
> Good person
is trying to be good ... no matter how hard ... once they do something evil, they still can be good person, there is no corelation.
> Good alignment person
is good ... meaning they dont have any other option ... once they do something Evil, they are no longer Good alignment ... or rather, it comes out that they never was, but that depends on interpretation.
This apply mostly on creatures that are not PC ...
Devils/angels, chromatic/metalic dragons, monsters ... etc.
Niara was talking about this in the past if i remember corectly, and according to my memory (aka, if i say something wrong here, its my fault) ... for example Good being like Angel, stops being Angel if they "somehow" manage to do something Evil ...
Like Zariel ... she was an Angel, she was corupted, she become Demon (or Devil, i dont remember wich of them is lawfull).
Bcs its impossible for Good Alignment creature to do Evil deeds and remain Good.

For PC, this is a little trickier.
In my interpretation, depends on how strickt you wish to be ... if you wish to be strictly good, you pick Good Alignment ... if you wish to be good, but also being ABLE to do evil, you pick Neutral.
Yes, im aware that even PC was picking alignment in previous editions.
Problem with PC is that we do have free will ... and there is no force on hell, heaven, or anywhere in between, including DM that could force us to act acordingly ... thats why Neutral was invented, bcs world is not black and white, and its much easier for Player to have both options.

And since even that didnt work, whole alignment concept was dumped lately.

Obviously, since alignment is just a game concept, it dont work as things do in reality (and we could find many other examples where the same apply) ...
So again obviously its impossible to read this litteraly ...
Its not like when Angel lifts an apple from the ground, they sudently become demon bcs that apple belong to someone, so this is basicaly steling > aka evil deed.

Are you familiar with concept of Karma?
All bad deeds you do stains yours, and there is no way to get rid of it ... otherwise it would loose any meaning ... you can only be pure, or not.

Neutrality implies not caring at all, or caring very little and putting morality beneath some other, probably more personal purpose or goal.
No, it dont ...
It actually have litteraly nothing to do with your caring.
It only states that you are able to do both, either, or neither.

Think about it as real world politic ...
You can be Left Wing, or Right Wing ... those are Good and Evil ... and yes, i purposefully dont state wich is wich. :P

But then there are people who like some ideology of Right ... but not all of it ...
Then there are people who like some ideology of Left ... but not all of it ...
Then there are people who like some parts of both ... and trying to make it somehow work together ...
Then there are people who thinks that each should be use in different situation ... so they switch basicaly ...
And im quite sure that there are also people who thinks both is wrong, and whole system should be destroyed and build anew differently ...
All that, is Neutral.

All that, and much more.
Usualy you can hear that there is 256 different shades of Grey ... while in fact, its more like infinite.

Absence of White, is Black > Evil (or Good, dont really matter in this example.)
Absence of Black, is White > Good (or Evil, dont really matter in this example.)
And
everything in between, is Neutral.
You can be a good person and falter in your morality. You can be an evil person and still do something nice.
The problem here is, that moral questions are matter of perception.
You stole someone property ... nod a good deed, huh?
> Unless you give that money to poor family who is trying to cure their child.
You wipe out Kobold tribe, that was attacking helpless civilians ... very good deed, huh?
> Unless that vilage was build on that tribe home, and they were just defending their own.
You kill a person in cold heart, not having any regrets ... a evil deed, right?
> Unless its a Hitler.

And so on, and so on ...
As i said, its a game construct ... it dont work well on complex matters, it was created back in days where Bad Guys were bad, and heroes were good ... nobody bothered back then to think about if this Raider gang leader was loving father, or if this benevolent king dont have too high taxes.

Just look at paladins. They can falter and break their oaths, but that doesn't make them neutral. It makes them flawed and mortal.
Breaking an oath =/= betraying your alignment.
Take the example from game itself ... our "first oathbreaker" (that NPC we meet in our camp, when we break our oath) told us that his oath was to follow his master ... and when his master gave an order he was unable to follow (i believe it was about killing some willagers), he turned against him and slain him instead.
That is exactly the point here!
Our Oathbreaker NPC was in position, when he could either break his oath, or change his alignment ...
And we know wich he picked.

Also, as stated abowe ... there is problem with quibble.
If your Good Alignment oath is "to protect weak" ... who is weak?
Is 50 willagers with forks, shovels and flails weaker than 20 malnourished goblins with rusty knives and some slings?
And yet, no DM i know would call you Oathbreaker for wiping them out for those "poor civilians".

A basically good person who's stuck in a bad plae in life and ends up having to murder someone doesn't automatically become neutral.
Indeed ...
But there is important part: "
having to".
You see Alignment usualy (to my knowledge) only apply on your willing choices, you can be purest Paladin in the world, and your very Good deeds may still cause some inocent victims, even if indirectly.
But that dont affect you.
You can end up "having to end up somoene life" ... and it dont affect your alignment.
Only if there was different option(s) ... but you DECIDED to murder them anyway (and it dont really matter why you did it) ... THEN your alignment is affected.
Or a slightly different example. A morally good person who has a bad temper. Say they get into a drunken brawl and kill someone ...
If you think that a good person who falters is actually a neutral person (and that is the implication of your claim whatever you explicitly say, if you don't mean that then you should be clearer
I dont think i implied such thing ...
As i say, and i believe i say it much clearer now ... what matters is your choice.
- You go to brawl, someone trips, falls on a table, break his neck and is dead ... not your choice.
- You go to brawl, grab someone head and twist it to break his neck ... your choice.
The person is just as dead in both cases ... both cases fits your example of ill tempered person in tavern brawl ... but only one of them affects their alignment, the one where they CHOOSE.

Alignment is actually quite poetic when you think about it ...
All beings that are bond by its rules, have so strong will, so it can reshape essence of their own being. :3
Have you seen the TV show Lucifer, with Tom Ellis?
Simmilar stuff.

how can you possibly get a corruption arc like you've advocated for?
Quite easily i would dare to claim.
Shadowheart ...
Lets say we find out that she indeed was a Selunite before they wipe her memory ...
We offer help with her revenge (i imagine she would be rightfully angry) ... but then we point out that even tho she was not her whole life devoted to Shar, she was for last *
insert time here* ... we can also point out that Selune never actually helped her during that time, feed her desperation ... we point out what kind of power she had during that time and help her ebrace her new self (no matter if in the name of Shar, or some completely different deity).
Result?
Evil Shadowheart.

Wyll ...
We will push him to make all those bad deeds among the way, he allready do (like torturing that guy, or massacring those goblins for personal feel of satisfaction) ...
During his personal quest we get several options to either point out that this isnt the way a hero should act (leading him to redemption) ... or we keep our mouths shut, and on the contrary encourage him to only hold a facade for public, while actually he can do anything he want, any time he want, anyhow he want.
Result?
Evil Wyll.

Gale ...
Gale hoards magical artefacts and knowledge, right not its in order to keep his condition from harming any inocents around (even tho potentialy prety far around) him ...
During his personal quests we find out that there may be a way to actually USE that nethereese orb in his chest, weaponize it if you wish ... and so we subtly guide Gale towards this goal ... keep all the artefacts, knowledge and power, and let the orb feed on other Mages instead.
There is even potential for grand finale, where Gale would reject Mistra for good, and decided to pleage his loayality to Shar, in her quest to replace Weave with Shadow Weave ... bcs nothing makes people more bitter and lustfull for revenge, than broken heart. :3
Result?
VEEERY Evil Gale.

See?

We would only guide them ... but they would need the ultimate choice themselves.

And regarding Astarion, I think he takes too much pleasure in sadism and petty cruelty for him to not be evil, but I do think that given his circumstances, it's not outrageous that he could be coaxed out of it. He's been the victim of two centuries of trauma, that'll warp anyone, but trauma can be worked through with time and care.
I dont really think this is relevant ...
Doesnt matter if he become Evil, was raised Evil, was born Evil, or just chose to be Evil despite all positive influence he had. Important part is that he IS right now.

I think any redemption from Astarion would require not just romantic love either. Romance or not, what Astarion needs is someone to support him and have his back, yes, but also someone to limit his excesses and lashing out. More than a lover, Astarion needs a friend and role model. And even then, it may well come out to naught.
I dunno, maybe im just bitter, but Astarion is in my eyes perfect example of "i can fix him" guy ...
People like him, bcs of his look, his voice, his charm and his "sad backstory" (completely forgotting that he MAY have lied, it wouldnt be his first time after all) ... and are completely willing to ignore all red flags, until (and sometimes even during and after) it bites them in the ass (or neck).

To me, this is almost a trigger.

And maybe for my own bitterness, i just wish for Astarion to have seemingly redemption arc, where ... in the end ... he would reveal that this was all just an act on his part, just to get rid of Cazador and taste his blood ... and now, when he is free, and become actually more powerfull than any vampire LORD before him (bcs he had his tadpole that gives him additional powers) ... you are no longer usefull to him.
And that would be the situation when he would attack you.
I have no problem with redemption arcs in general ...
I just dont like this modern trend where "everyone can be saved" ... i like darker fantasy settings, where some people simply didnt want to be saved.

But that is matter of prefferences.
