What should I do? Will he get violent?

I was seeing this guy. I'm 24 he is 31. He said he didn't want to date me bc I was immature and inexperience in life. He said i was crazy and had head problems to fix before I could be with anyone. Everytime I try to break it off he wouldn't let me. He ended up breaking it off and telling me to delete his number and other hurtful things. We work together and I told him to stay away from me. He did not like the sound of that. After he ended it with me, at work he prolongs any conversation with me. He stares at me or make noises to get my attention. Other days he avoid me. I also believe he stole my phone. Oh and he had a gf the entire time that I didn't know about. Why is he being this way? What should I do? I've tried to talk to him about it but he won't let me.