Preemptive is a different word from defending because they mean different things. Preemptive violence is violence that happens before a threat exists because of the possibility of a threat.

So shooting your neighbor on Tuesday because think he might be violent on Wednesday. If on Wednesday he shows up and makes verbal threats of imminent violence, responding to the threat of violence by being violent first would be self defense and not preemptive because the threat actually exists at the time. Timing and context matter, not who literally who gets off the first shot/punch/violent act.