You can't call the police. I mean you can, but they probably won't do anything. Depending on what state you are in, they won't even do anything if someone walks up to your face and threaten to do you harm. They have to wait until it has actually been done. (Massachusetts is great...).
Another downer: all you can do is report him in the game. Which will just ban his chat for a few games. OR ban him completely! Which sounds great.. except he can just make another character and come back... so that's pretty much a waste of time.
Despite the shitty options, there is no need to worry. Probably just some 10 year old talking a big game from the safety of his room, behind a computer screen. You can't take these things seriously. I would advise (if you don't like that kind of attitude) to find another game. Most of the community is like that. I know there will be 10 people or so saying I'm full of crap... but just look at all the posts talking about it. All the videos. League has a reputation as being the most toxic game, and it didn't get that title for no reason.