I played evil (female Drow from the Lolth culture), but not insane. So all-in-all, there is not much I would do differently from my future combative good character.
* Hints about what is the Absolute, or even just the basic obvious Mind Flayer relation in your own brain, means that a sane evil character will certainly never help the Absolute! You would need 7 or lower Intelligence and Wisdom, or be insane, to join the Absolute. Most evil characters do not want to be tools or subservient puppets. Your evil companions (Laezel and Shadowheart) have their good reasons to fight Absolute pawns with all they have got.
As a Drow, whether I serve Lolth sincerely or not, any Drow controlled by the Absolute will die ... If I become an Absolute tool, I would expect others to give me a quick death.
* Halsin is better help than that loony shadow druid (she is a rather petty evil). (And, as a female Drow, I found the Tiefling girl stealing the statue an awesome and cunning kid!)
* I have nothing against the Tieflings and I would rather slaughter the goblins that are pawns of the Absolute. I don't need to help the Tieflings either, but not much point in making random enemies: what is to gain? Everything to gain destroying Absolute nutcases (or victims really, as I believe them to be basically controlled).
So other than a few side quests and extra violence, my sane evil characters will not do much differently than any violent good character. Playing a non-violent good though would be a different playthrough.
EDIT : this may come as a surprise, but I am not criticizing. Just the rationale of my somewhat evil, but not stupid, Drow. I do not mind how it is.
Can agree with a lot of this.
Current evil plays seem rather lacking. I don't play evil as murderhobo and it kind of felt like this is the encouraged path which is sad. It could be that there is better content for it later on that we won't see in Act 1 but this is not guaranteed, hopefully they can get enough feedback and change things somewhat.
How I played two characters:
For my favourite evil character I played so far, I actually saved Halsin because logically he was the best option at that time to deal with the tadpole. He would die later if useless. Kagha and her friends died because they were near enough to Nettie that they would have discovered her body. Didn't care about the tieflings (who lived because I never spoke to Kagha, only killed her), but at least they were respectful enough. Don't trust this Absolute stuff, so the goblins and Minthara died. Don't make bargains with devils so refused Raphael's "help". Killed Ethel for obvious reasons, game wouldn't let me kill Mayrina for some reason (didn't like how she spoke to me). Didn't care about Volo being a prisoner so never spoke to him or saved him. Let Omeluum try his method but he may find a knife at his back when the opportunity presents itself since he made the damn crap stronger. Had no issue with Myconids since they are not in the way and they could be useful later, (also after accidentally stumbling across Baelen who saved himself I found out how awesome Derryth Bonecloak is and really don't want to piss her off. ).
The other evil character I haven't finished the current content with yet sided with Minthara partially due to conditioning and also because of the promise of power with the Absolute. Tieflings and druids had already fought so it didn't matter about giving away the location. Sees Minthara as an ally that can be manipulated whenever necessary after she showed her weakness in the camp scene. This one would want to use the tadpole if possible and not get rid of it unless necessary. Probably will refuse Omeluum's help in case he is trying to manipulate the tadpole into doing what it is supposed to be doing.
I would play with another character and maybe only accept Raphael's help but we don't have any more content with him involved to properly test.