I believe that GLaDOS's body is cursed so that whoever is in it is evil

[Maximus10113X]

Think about it. When you defeat GLaDOS in Portal 2, Wheatley becomes evil when he enters her body. You could say that its just the power going to his head, but another thing is that when GLaDOS gets put in the potato, she turns good. But that could just be because she wants her body back and the only way she's gonna get it is by working with Chell. Do you think its the body thats the problem? Or something else that I'm missing. Or maybe its just a coincidence.

[Maximus10113X]

Maybe Wheatley turned evil because he's programmed to be a moron and to be not good at anything

[Siluva]

GLaDOS was evil from the start, since the very moment she got powered on up she flooded the whole center with neurotoxins, which led scientist to make those cores and attach them to her to "temper" her.

Now the reason why GLaDOS is evil probably comes from the fact that she comes from Caroline, and as Cave mentionned there where high chances that Caroline didn't wanted to be implemented (or copied idk the process they used) to become a super AI.
So maybe she is either angry at everyone and taking revenge or something when wrong during her programation.

Now there is probably other external factors, because GLaDOS mention she heard the voices of the cores in her head her entire life (which probly made her crazier than before) alongside an harcoded routine that force her to make experiment with a reward system trough stimuly.

The same hardcoded routine that prevent her to help you with how to solve the test chamber and drove Wheatley addict of those stimulis, because afterall, he is just a core programmed to be dumb, not to hold the power of an entire automated research center.

Personally that's how I see it.

[Maximus10113X]

Good point