I assumed, the evil motivation to kill Ketheric would be because you want to take his place in the ladder of power or something like that?
The "evil motivation" in act 1 and 2 is that you're a retard.
You retardedly side with cultists, who are seeking you out and trying to steal the artifact, the only thing that's keeping you from brainwashing. Then you slaughter many innocent people with these cultists for no promised reward, just so you can infiltrate them.
After infiltrating them, you have another episode of retardation and venture down to Grymforge. There you save a brainwashed cultist Nere for (again) no promised reward, and get nothing for it. By the way, Nere dies off-screen later. Didn't have the money to pay the voice actor, I guess.
After that, you enter the Shadowlands, where you head into the Moonrise Towers to learn about your condition and find out who is the Absolute. You effortlessly infiltrate the cult, and then you are told to do yet another errand for them, but this time there's an actual promise of a reward - meeting the Absolute. Why would you want that? Well, there's no actually good reason for it.
In Moonrise towers you find out that your evil companion, Mintharra, has actually fallen from grace now, and is no longer in good standing with the cult. You rescue her and finally recruit her. Mintharra's motivations are no longer aligned with the cult, and she explicitly wants Ketheric dead for mistreating her. That probably should be your new motivation too.
Despite that, the game continuously gives you the options to keep doing that errand for the cult. And here's where things get really weird.
You can
1) Continue doing this stupid errand for the cult for the nonsensical reward of meeting the Absolute. Despite it being a very stupid thing to do, some effort is actually put into that path - there's voice lines, there's several well-animated unique cutscenes that you won't get on the good playthrough.
2) Destroy Ketheric's source of immortality by killing or freeing Nightsong. Both of these are identical to the good path, but with less voice lines and cutscenes, and absolutely no unique ones.
If you pick the path number 1, you meet Ketheric, sort of meet the Absolute, they finally find out that you're an infiltrator, and you get captured like a retard. Then you have to kill Ketheric. At no point your genius plan of infiltrating the cult pays off.
If you pick the path number 2, you're either back on the good path, or back on the "good path, but with no good content". You go to the tower, have a very short conversation with Z'rell, kill everyone on the way to the top same as you do on the good path, but with no harpers or Jaheira, and then you fight Ketheric as you would on the good path. Again, at no point your genius plan of infiltrating the cult pays off.
Now, this was a lot of text, but let me sum it up. Larian devoted a lot of time, money and attention to making an evil path that is actually the retard path, where you keep infiltrating the bad guys for no reason and no reward. At no point you learn some information, find some weaknesses, subvert their organization from within or get the opportunity to do anything clever. You insert yourself into a dangerous situation for no reason (like a retard), and then you get captured (like a retard), and then you get nothing out of it (like a retard).
Now, Larian actually broke new ground. There are very few RPGs where
this amount of effort is dedicated to opportunities to make a fool out of yourself. And I dare say, that all this effort could be diverted into something else. Like an evil path that's actually cleverly written, or maybe just scrap the evil path altogether and make... I don't know, the Upper City or something.
. In such moment i can barely contain my laugh, its just so hard to treat it seriously, especially if such manlet looks like a woman with dyke haircut.
