*One possibility is that Yandere-kun - exactly like his female equivalent - is an emotionless boy who feels no reason to live...until he meets Senpai, and finally begins experiencing the same emotions and sensations that everyone else can experience. Yandere-kun would seek to eliminate anyone who threatens to take Senpai away from him, because Yandere-kun doesn't want to lose the one thing that makes him feel alive.

Another possibility is that Yandere-kun is a naturally sadistic person who requires an obsession to give his life meaning. He has marked a girl to be "his" - his property, his possession. He's hunting down anyone who flirts with Senpai because it's like a game to him...a game of death.*

walking towards you but then gets nervous and backs away

"I can't... He mine...but I'm to nervous to get near him..."