Thinking about two random examples that might work, but they might not fit the theme very well. Anyway, here goes:
CSI 4.10 "Coming of Rage"--where the crazy girl takes part in killing the other kid with a hammer for fun. She seems like a victim at first but is later revealed to be a total nutjob, and she basically tells Sara (I think?) that she'll get off for looking innocent in court and exploiting the fact that she doesn't have parents.
CSI: Miami 4.04 "48 Hours to Life"--an innocent kid gets put in jail, but before they can clear him, he is raped by his cellmate and kills him, then lies about what happened out of shame. He didn't really turn evil, but the 'society' in jail changed him, I guess.
Not sure if those will help, but they're the only ones I could think of. You could scroll through the episode guides linked at the top of the forum--most of the later episodes have reviews you can click to remind yourself what happened and whatnot.