I constantly think about chopping people up with a meat cleaver and eating them. what is my mental illness?

Hard to say....: but that's what psychiatrists are paid for! If you really want to know, I would definitely schedule an appointment with a psychiatrist.

If you are serious: The name is utterly unimportant. The intrusive thoughts are very disturbing and you need to find a safe place. After you and people near you are not in danger, there will be time to find a name for the 'thing'. If you're not serious, please understand that this service is. We are here to help, and some folks are hurting badly. Call a local help line or ask a person you trust to do it.