Given that you are sixteen, ordinarily I'd say to tell your parents and see if they will take you to see a therapist or psychiatrist, but the things you said are pretty extreme, so I might even go so far as to say go to the ER and tell them what's going on. Better to go to the ER now than go to prison for doing something terrible when you know, now, that you have a problem.

Just walk in and tell the receptionist you are mentally unstable and need to see a doctor. When you tell them that you are having thoughts of harming others, they should keep you and get you in to see a doctor. I hope.

If and or when you do see a doctor, be completely honest with them about everything going on.