You have the choice to turn off the computer. You do not have a right to threaten to kill someone. What does your age and you "wanna live" have to do with anything? The police aren't going to kill you!
3-4 weeks ago i got in trouble for saying i was gunna kill an anonymous cyberbully(not literally) and the cops got involved and now idk what to do now im 15 and still wanna live?
Gone are the days when you could say anything and get away. Now a days you either get sued or screwed. One needs to moderate their behaviour outside.
If that really happened, you need a lawyer.
just apologize and tell thern you werent serious about it