I think Quinn isnt so much a bad person, she just hasnt got the luxury to be an outstanding member of society...
Basicly she is in so much shit she can only do whatever she has been doing to survive as she has been doing so from young age, just her and maybe her brother against the world... She will not have had alot trust in the world growing up from parents with a serious drug addiction.... but she learned how to survive by manipulating instead of just being a victim.
Yes, Quinn is a major bad influence in the story, she sells drugs, sex, even fake scholarships, she most likely posted the Cathy picture on rooster so she could steal the money from the DIK vault during that party...
But at the end of the line she is just one tough survivor against the system in a world that hasnt been easy on her so far...
Can she change? or is it the world (or the MC) that has to show her she can trust others again?
Society grows people like Quinn if it keeps acting like Tybalt thinks