Tell your parents, the sooner the better.
I think I know a psychopath. He's 13 (so am I), he dreams about laughing and killing people, he says he wants to grow up to be an assassin and today he confessed he wanted to actually kill someone. How do I stop him from hurting anyone?
Yes, you need to tell someone about that young man's ideas - what he's saying to you is not good at all.
I too would recommend informing someone. Ether your parents or a teacher. This guy sounds like he many need help.
However, for your own peace of mind I would also say that 13 year olds tend to say and do strange things to impress their peers, and it's possible this boy is saying these extreme things to show off.
Nevertheless, it's best to be safe than sorry. It seems like not a week goes by without another tragedy or school shooting making the news, and often it does feel like the whole thing could have been avoided if the troubled youths involved had received the right kind of support earlier on.