I try very hard to see your point, and at some stage I thought perhaps in the realms it would morally good while not in the real world and vise versa, but no matter how much I think of that I can't find it "good". If you are talking about self defense it is something else entirely. But if you kill someone on sight because he might be evil* you are evil. If any of the good aligned Gods allow this kind of behaviour it is more of problem of the settings than of your train of thought.
*Not talking about situations like war zone or something like that.
It's not about being good it's about being acceptable. If you go to a forest and you see a hag, virtually no-one is going to judge you for killing her and then bragging about it, because it is understood that hags are evil creatures that eat babies and make people's live miserable. The same way, if you kill a red dragon without said dragon first initiating combat, no-one will call you an irredeemable villain for it, because red dragons are cruel and evil beasts (even though they are sentient). Now here we arrive at drow. It is understood in the realms that the drow are evil, cruel, and seek the death of all surfacers to please their evil goddess. Killing a drow wouldn't be considered a crime pretty much anywhere, because the tales about the drow being evil are actually true, and the chance that you killed a "good drow" is virtually zero. Keep in mind that you are not killing the drow because it's evil, you are killing it because you know that drows seek to harm you or others on the surface.
The real world is very different, because in the real world we are all humans, God's existence can't be proven with material evidence and most importantly, we are the only sentient race on the planet. Now, as for good aligned gods allowing this or that kind of behaviour, it's up to them. But there are most certainly plenty of neutral gods who aren't going to make a fuss about killing some drow or tiefling, and since your afterlife ultimately depends on faithfully serving your deity, everyone is incentivized to pick a deity whose tenets they can live up to, and whose afterlife they find to be appealing. Kagha really isn't going to care that you think she is evil, because what she did doesn't really violate Silvanus. She will just die one day and end up in her chosen afterlife. Your judgements will mean nothing to her, because frankly, she has no reason to care for it.
