Someone threatened to delete my character for killing a boss?

see ultimately Arxus it boils down to this^. It's in the Terms of Service that you have checked and signed for. You can't blame the "Who reads the ToS" argument because even if you were signing a contract to work, you are bound by the Employment Agreement you will have. So be sure to read it.

Nevertheless, submit a ban appeal, otherwise yes 3 days will come and go in a heartbeat. Or worse case, just be done with the server and move on.