I agree with Brofist12342! Mary was just a- a puppet of her world. She couldn't do anything! She never had a friend, she probably never slept either ( like you said )! She only wanted to be with her father... Guertena...
"...the reason why is because I don't think she intended to KILL Garry, she wanted to leave with Ib because she wanted a friend and she wasn't very fond of Garry because he found out she was just another painting by Guertena..."
This absolutely correct! What if she wanted to go with both of them but had to pick 1? Yes, she probably didn't ever want to kill Gary, she just wanted him out of the way to just be with Ib.
Right when I saw the part where Mary was plucking the petals off of Garry's rose, I was like," Oh Mary... If you kill Garry Ib will be sad. If Ib is sad about you murdering Gary, she wont want to go with you. I she doesn't want to go with you, you will be depressed... If you're depressed, you'll get angry at Ib for not wanting to go with you. When you get mad at Ib for not wanting to go with you, you'll kill her. If you kill her, you'll be lonely... AGAIN". (lol)
To be honest, Mary is actually my favorite character. The reason why is because I don't think she intended to KILL Garry, she wanted to leave with Ib because (in the game when Marry starts following you around after stabbing the head, she'll say "I've always wanted a girl like em to be my friend" and stuff like that ) she wanted a friend and she wasn't very fond of Garry because he found out she was just another painting by Guertena. She was lonely and she was locked up in that world for who knows how long. Never aging, can't eat, drink, I don't even think she could sleep so she went a little crazy. But trust me, if you were locked up like that, you probably would've lost it as well. She just wanted to go outside And I don't know how, but I feel like I can relate to that That's clearly why she's my favorite :3