He's just a misunderstood bunny rabbit that killed the purple guy to set the souls of the children free. Everyone thinks he's a bad guy, but I think he's a good guy. He probably doesn't kill you when he finds you, probably just scares the everliving daylights out of you and leaves like nothing happened. :3
(By the way, the whole thing with him killing purple guy is my theory, which is that Springtrap actually has some cognitive thought and remembered that purple guy used him to kill the 5 {6?} kids. Remembering this, when purple guy tried to escape the souls of the children by hiding inside Springtrap, Springtrap released the springlocks himself, killing purple guy. I wanna hug Springtrap so much because I know that he would do the same for me ^u^)