Mass Effect has never really allowed being evil for the sake of being evil, which just comes off as cartoonish anyway.
The problem with having both good and evil characters in a story is that they're often in conflict as to what they would do, but since this is a video game the questline is going to naturally go towards one path because it's an insane amount of work to allow for two wildly divergent paths.
Even in KotoR if I want to be pure evil I can't just hand Bastilla over to the Sith at the start of the game. I have to go through most of the game doing what the Jedi tell me to so I can do a last minute switch to the Sith. It only works for KotoR because that's kind of the standard for Star Wars morality.