Mental Health / Worried for my own security?

I have a friend who tends to have random spastic acts of violence. Today, he randomly slammed the corner of his iPad into my kneecap and damaged it, afterwards he acted like he didn't even know it happened. He hits the table sometimes for no apparent reason and throws his things across the room or slams them on the table as well. There are times where he's chill and silent or says something funny, so I don't want to just abandon him as a friend. I don't know what to do and I'm afraid that he'll hurt me or my friends. Side note: Sometimes the violence gets so bad he tries to bend all of my fingers back to break them, or he tries to break my arm. He tries to stab me in the side using a mechanical pencil, etc.