Why did Jasper Hale stop hunting humans and became a "vegetarian" vampire?

yes. you spelled vegetarian right. haha

and he stopped because he always could feel the emotions his "victims" were feeling before he destroyed them. The pain was too much to bare. And when he wandered around, there Alice was...waiting there for him. "For the first time in over a century, I felt hope"

That is one of the reasons why I pag-ibig Alice and Jasper. He changed for her. He knew what he was doing was wrong, but his vampire instincts tell him to kill. He did not control Alice when they had met. He saw her waiting for him, and from that moment on, they knew they had found a mate.

alice had a vision of where he would b and he joined the cullen family. and since his new "family" were "vegetarians" he became one too. also it could of been about his past and how he wanted to change.
Ps your spelled vegetarian right

He stoped n became vegeatarion because he felt the pain n horror as he killed his victims
he also changed for Alice becuase she was part of the Cullens n they were vegetarians so to be with her he had to change his ways