Hunting Annabelle

I feel disappointed with this because it actually started out so good, the setting, the character work absolutely fascinated me at the beginning. 
I thought Sean's POV was done so well, and how his mental illness added a psychological aspect to the book.
Also I read another book from this author which is called You Can Trust Me, and I loved that one, so I was excited to read her debut.

Later on, though, the part of the book where Sean is ‘hunting annabelle' was so cheesy, it read very YA, kinda like a Nancy Drew mystery. 

The plot twist definitely caught me off guard, but I'm still confused.Sean only murdered 2 women, at the most. Annabelle is literally on her 28th. Why is she so obsessed with killing him and being all vigilante? If anything he should go vigilante on her. And there's some scenes at the end where she's gonna murder him and he literally jumps her and starts making out with her...? And all her apparent hate for him disappears? The guy she's been planning to kill for years, and she's just okay with being with him instead? It makes no sense.

Also the twisted love story aspect of the storyline, I didn't particularly enjoy it. 
Overall this was a very linear, basic thriller. I don't think it's doing anything special.