Ren'ai Hakyoku Ukeoinin

Main character is a student helping at the detective office of his uncle. One day he receives a case to help a man divorce with his low quality wife. But no one expected the man to be kissing another man in secret! Protagonist's mindset gets greatly influenced by this fact - from now on he starts to manipulate minds of people. He gets either love or madness.

[From vndbreview]