To me, "evil" character isn't character who murders everything cute and nice, and definitely not a murder-hobo, but as was said above, an egotist. Egotist who is cynical, and hopefully intelligent, someone who manipulates, twists and corrupts.
In my eyes, "evil" character would find the way to actually return back to the ranks of the Chosen, and use their plan, take Oryn's place, perhaps, because she seems to be the most aimless and useless member of the group that doesn't DO anything of worth aside of running around and killing people. Objectively, Oryn is a bad villain and a terrible rep for Bhaal cult, because if you look at Amelyssan from TOB, real priestess of Bhaal could be a manipulative, scheming, clever character who would gaslight you into thinking you are with an ally. With Oryn we have a slasher-movie tier antagonist, who uses her limitless power of transformation to get the cheap scare out of you, and nothing more.
Where's my "Oryn turned into Tav, committed crimes, and now good guys think you're a murderer and you need to escape prison and hide and try to prove your innocence" plotline....
Anyway, beside the point, to me evil character would find the way to work with the most immoral elements and benefit from them. If Gortash is as genuine as game tells you he is - why not side with him? Why not let him rule, while I do my cult stuff? If KT wants his babygirl back - let him have her, why railroad me into killing her, it's so pointless when there's so many other innocents to kill (Bhaal is very unreasonable and silly as a deity, as well as Shar).
But most importantly, it's about flavor. It's about you feeling like a bad guy, like a villain, colors tinting. Maybe I don't stay at the cozy inn, and I stay in the sewers or in a crypt or I take over some other bad guy's mansion and squat there. Maybe my companions are not indifferent husks, but actually get corrupted along and we're like this motley crew of villains at this point? I think that's what people want from evil path.
The problem of "logically i'm not 100% good and my actions are selfish (in my head)" is that within the game you are still framed as a hero up until the end. There's no cool presentation to go along with it.
Heck, in Rogue Trader, when you turn heretic, your ship is full of chains, cages and burning flames. I'd want THAT for my Bhaal cultist, not a neat little inn.
