I have a friend who is...well, troubled. Last night, on a social network site he said that he: "Wants to kill something and listen to it scream" A friend wants to call the police. Should we?

Yes. And i shall tell you why, threats like that could be the factor of something going on deep inside his head. Take it from me i know all about mental health, not only because i took a course in it, but also because i suffer from it.
Your friend seems to have a twisted way of wording things. Yes call the police as it needs to be sorted, he could actually do something and then where will you be?
You will be guilty of something.

But for the long term - i suggest you go see you doctor and tell him of your worries and how you could handle such behaviour as well as give you pamphlets on the odd behaviour or put you into contact with a mental health team. Depending on how old you and your friend are it could be something to do at home. Is your friend being neglected> Is he often dirty, quiet, wolfs down his food as though he's never eaten before, have mysterious bruises he's quick to give an explanation to. Talking to your parents may help, your teachers in school a boss in work.
Calling 991 (999) may not be the best idea. Call a phone number operative ( a switch board that gives you numbers in your area) and ask for your local police station. Depending on the severity of the case, calling the emergency services about a friend who 'could' do something may not be welcome, but calling your local police station won't do any harm.
You could also try sitting your friend down with a coke or tea or even a beer and ask how he's feeling, about home life and just support him in his time of need.

let him go , he is not a freind. he may want it, but he will get a rude shock when its his own screams that are heard. for all will know of his intent, but he wants to find someone to help him for he isnt capable of doing anything on his own. but i do hope he does do something for , when he hhas done what he has wanted, he will be found and he will become a bitch for many, and he will wish that he would be able to sign the sorry book.but somewhere no one will hear his screams, but forgive him, and i do hope you are more carfull, and that you have limited details available of you. he wanted to impress you. but you are protected, so move on and have fun and do be carefull;-]