why (spoilers)

I've only watched it once so maybe I need to listen to the dialogue after the big reveal again, but I don't get what Seth was hoping to accomplish - how would she be "rehabilitated" and how would he even know if she was. She could just tell him what he wanted to hear to get out of the cage, then go right back to it.
I don't get why he didn't contact the authorities with his concerns - OK the diary would be flimsy evidence but after seeing what she did to the homeless man there must have been evidence. Moreover, the film was set in 2016 or shortly before so it's implausible that he couldn't have filmed her crimes on a smart-phone without her knowing.
I get that he was mentally unstable to begin with, his feelings for her made him worse and affected his judgement. Plus we wouldn't have much of a movie! But I'm wondering if this might have made a better stalker movie with this twist - albeit the twist would come earlier as we'd find out at the same time as Seth