People see Mary as more innocent than Clark, not as much of a bad person, but I'm not sure that's the case. She's less obviously destructive, but...
Mary's treatment was a loop I think, as in, she probably used the same style treatment for everyone (as suggested by her tapes, being literal copies of her message.) Clark came to her for help, and she kept on the same treatments over and over that clearly weren't working. In particular, she kept roleplaying as his wife, who (he thinks) took everything from him, making him relive a terrible memory.
Of course he hates the roleplay! It's just bad memories and positioning himself as the bad guy. And maybe he was, but why would anyone like that? Mary doesn't ever seem to even give him a little slack about what happened, when if what Clark says is true, it would have been a very stressful situation to live with.
But she forces him to roleplay and relive a terrible night multiple times, so in the dinner scene he forces her to do the opposite: roleplay again and finally see him from his own perspective. Mary wanted to act as his wife, so he rubs it in by forcing the hair on her to make her realise how horrible the whole memory/situation is.
This isn't to say that Clark isn't shirking responsibility (he is) and that he's actually a good man (he isn't), but Mary didn't really try to engage with Clark despite him begging her too. Clark reached out to Mary for help and her treatment only made it worse. It's no wonder he took a chance to escape when he could. Mary was right to be terrified when entering the Backrooms, being a reflection of his mind, because she hadn't really entered it before.
I think she does it because of her mother. Her mother was delusional, and you can't entertain those delusions at all when interacting with them. Clark is angry and shirks responsibility, but he isn't delusional. Mary, stuck in her own loop, treated him like she would her mother. At best her blind treatment of Clark just didn't help him, but at worst it actively made him worse.
It's probably (at least partially) why she smiles at the end during the interview/interrogation. She's finally understanding what it's like to have someone try to analyse you without really engaging. It's patronising and probably going to make everything worse. We even see a Still Life of her sitting utterly terrified and distressed in that exact chair, suggesting that was how Mary was feeling, while all that Phil had to offer was questions he already knew the answers to and a vague threat that she'll be taken away.
I don't think Mary is a good person, honestly. She's not that different from Clark in the end, although her method of coping is at least (usually) more helpful than his. But it's more destructive when it isn't, because as a therapist she works with damaged people and helping the wrong way leads to real harm.
i couldnβt figure out why tf maryβs phase into the
#backrooms was presented so much more intensely than clarkβs but iβm now realizing itβs because she quite literally enters the headspace of her patient; something sheβs never given herself permission to do. 1/2