[m]
OK I have been looking for awhile now a manga I forgot the name of.
Pretty much there is a high school (I believe he is anyways) boy who loves this girl who is only attracted to killers, its like a special power of hers and her dad uses it since he's some kinda cop or something. He works for her dad and they have been friends for awhile, she can't seem to pick up that he is a bad guy too. That's as far as I got before I lost the manga.

Is it Boku to Senpai no Tekken Kousai?

I just read abit of it now. I think that is it! Thank you so much!