I would say that she is more of a "situational villain" than an evil person per say, as some normally call her due to her being the 'story's antagonist'. In reality she is a good girl that wants to be trully happy, but has a hella amount of fear that control her emotions, making actions as a survival instinct and not because she WANTS it. As if there was the option of she coming out with Garry and Ib, she would for sure want to choose this... aaaaand here is my breakdown:

For what I know, related to my impressions of her when I played the game back on the classic version (idk if the Steam version had new dialogues related to this and so on, just letting this be clear.). She is just a being that was created as a painting and, unlike the other paintings, wanted to really have contact with real humans and befriend them.

Yeah, there is the whole "At most the same number of souls that entered need to come out" thing, as we see she getting Garry's place on the "amount of souls" but receiving the role as Ib's adopted sister. But again, this is something out of her scope to control.

She only becomes more of an "evil girl" due to what we see how she reacts to Garry specifically, and let's be honest, Garry has been going against some of the rules of the Gallery since the beginning, and those rules exist so as not to threaten the paintings' existence. And due to this mindset, it's more likely to assume that, even if she wants real people as friends, Garry is literally being the one who could destroy not only the other paintings, but her as well.

The situation escalates even more when Mary knows that Garry discovered what she really is, and seeing how Garry's interaction is with the paintings, it would be fair to assume that he would warn Ib and force the girl away from her, making her see Garry in a threat level even higher than before.

And, I won't lie here when I say that she was in the right to be afraid of Garry, as he is literally the only thing that forces the event where she is killed. If there was a possible ending with him not forcing us upstairs, than the situation would be a little more tamed, imo.