Fadaar
That guy
Evil playthrough makes some encounters easier and some harder.
Act 2 is harder. You need to take out Last Light losing the Harper vendor, you never see the Tiefling vendor either. Then in Moonrise Tower you have to fight the place without allies (I forsaw this and killed the place bit by bit before Nightsong), you have to fight the Drider with Ketheric and the have to fight Ketheric the second time with Isobel on his side.
Act 3 is easier, House of Grief and Temple of Bhaal (as Dark Urge) are solo battles and you can skip all three of the Gortash quests if you want. I assume the end sequence is harder as you'll have less allies, but not done that as evil yet.
When I did my only finished playthrough I cleared the entirety of Moonrise other than the vendor(s?) before even doing the majority of the rest of act 2. For being a group of goody two shoes I sure was murderous.