Tom Tucker: Hey Ollie, hows that evil playthrough?
Ollie: IT'S BAD!
Please note that I am focusing primarily on the grove arc here.
With that out of the way, here we go, playing as a no holds barred antagonist is lame.
REALLY lame.
.
First: It is so much more expensive, the sheer costs of doing this are absurd. I burned a cubic ton of restorative items, scrolls, barrels, grease bottles, alchemists fires, GOLD, and revive scrolls on just the fights alone. Beyond that you end up butchering vendors so gold income and also takes a serious hit, on top of that you end up losing Wyll and Gale (if you don't pass a check that is, wyll leaves no matter what).
Second: The payout sucks! After everything I lost taking this especially dire path all I got in the end was the meager possessions of a bunch of refugees along with whatever you feel like prying off the druids sorry carcasses. OH! There's also a sex scene with Minthara but, that is a very fleeting "reward" indeed as she tries to murder shortly afterwards and tells you not to go back to the goblin camp even if she doesn't whack you, so thats one less vendor to buy and sell with. In short Drow boobs are fleeting, lost companions and vendors are forever. It's just a bad deal on the whole.
Third: It's bloody difficult. First you gotta pass a lot of checks in order to do the evil rout, then you need to go very out of your way through a lot of dialogue, then you need to NOT take any of the two dozen outs they give you in case you suddenly don't want to be an ABSOLUTE scumbag then after all that you ultimately get to be utterly wrecked by a swarm of very angry druids who will ABSOLUTEly beat the tar out of you and a handful of goblins.
Last: Going 100% hardcore and taking every evil possible action that can be taken, is stupid to the point of actively shooting yourself in the foot... repeatedly.