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.
