Both work. A cop killer can be killer of cops or a killer who happens to also be a cop. I like it over killer cop for two reasons
it implies that the killer part is the essence and the cop part is external to that
it replaces a term meant to set apart a category of killers for being worse because they kill cops (as if “blue lives matter” more than all others) with the many times more common killer that’s worse for being a cop.
Both work. A cop killer can be killer of cops or a killer who happens to also be a cop. I like it over killer cop for two reasons
it implies that the killer part is the essence and the cop part is external to that
it replaces a term meant to set apart a category of killers for being worse because they kill cops (as if “blue lives matter” more than all others) with the many times more common killer that’s worse for being a cop.