Why dude?
Nah, he didn't get the FMC hooked. She was already doing those drugs herself, for reasons that are a whole 'nother story. The game brought that up a bunch of times.
He didn't make her a slut at college either. She went down that road on her own; Mia just kinda cheered her on.
I believe your perception is skewed.
Mia is absolutely responsible for pushing Lacey in that direction, she admits it and she knows this, to the point of describing it as her burden, it is clearly stated by her in the dialogues.
She found a vulnerable and broken person, and she used her to have fun, to up the ante every single time.
She was recording all the stuff, do you think she was filming a documentary?
She was living Lacey sex life vicariously and having fun doing so, all the while hating MC because she felt he was stealing her toy attention.
If by cheering her on you mean being an enabler then yeah, she might not be directly responsible about the ketamine use, but she enabled it and enablers are often worse than users.
She enabled, pushed and cheered a self destruction ritual for 2 years.
For fun.
She wants to see Lacey's life crushing and burning down (this is stated by her), so she can be again in the position that she had in college.
She is trying to find a new woman for MC in Kelly, she played a "game" with MC for 3 months.
Now let me state the obvious, rape is horrible and that should not have happened to her, it is tragic.
Does that give her a free pass to be the monster she is? No.