This is just a guess, but it looks like the main reward for the Evil Murder Hobo playthrough is that you get to engage in combat much more often and that you have bigger and (more interesting? again, I'm guessing) battles. For instance: Tieflings vs. Druids, you vs. the winner of the grove, or Goblins vs. whomever is in the grove, then, if you are still feeling a bit bloodthirsty, you vs. goblins.

Which is an excellent gameplay reward that also ties in with the Baldur's Gate lore about gods of murder and bloodshed AND with the Illithid tadpole's tendencies to eat each other up before they ascend to your eye.