Foul language is permitted as there is a language filter. Turn it on if you don't want to read that language.
This game is all about killing people, as in any other game with real people there is a lot of trash talk. I wouldn't take it too seriously.
If you really feel your life is threatened then you should call 911. (Not advising that at all unless you know the person irl)
This game chats aren't recorded, and therefore you can't really prove that the player said anything at all, which means the devs would have to ban on your word and yours alone. Unless of course you have screen shots.
You however can email them and let them know if you like.
Games@chickenbrickstudios.comAny convos that take place outside the game ie gtalk, g+, palringo etc would have nothing to do with this game and a player can't really be banned for that.
Hopefully this one person didn't sour you from the game. Its been a lot of fun for a long time for many of us. I advise you go with the "/i name" option and not stress about it too much.