Hi, this is many things to me, it is the title of a song and a revolutionary saying, it should read like a declaration or slogan. This is going into print, as a moto or tattoo. Thanks in advance, Doc.
For ‘kill’, you can say interfice, interime or neca. Since the exhortation is no doubt to a crowd, the plural forms interficite, interimite or necate should probably be used.
‘The king’ is regem. You can put put before or after. Regem interficite, for example.
There is also the alternative of just saying that the king is to be killed. This is rex interficiendus/interimendus/necandus [est].