What makes someone a killer?

So I was reading an article for my Metaphysics class about Free Will and Determinism. As I was reading the question came to my mind, "does killing once make someone 'a killer'?" Now, think about it. If a pregnant woman kills a man who is in the act of physically attacking her and her unborn child would she be considered a killer? If a pregnant woman killed a man who had physically attacked her and her unborn child two days ago would she be considered a killer? If Mr. X killed a man immediately upon finding him in bed with Mrs. X would Mr. X be a killer? If Mr. X killed a man two days after finding him in bed with Mrs. X would Mr. X be a killer? If a man killed a person before being diagnosed with schizophrenia and receiving treatment would he be a killer? If a man with schizophrenia, while taking his medication, had a internal chemical lapse and killed a person would he be considered a killer? If a man with schizophrenia decided to go off his medication and killed a person would he be considered a killer?

What determines moral responsibility? Why would the killing of an attacker in the moment be considered self defense while the killing of an attacker twenty years after the crime be considered murder? Does moral responsibility step out the moment we can't see the outcome of our future? Even then, if the pregnant woman were being attacked by her boyfriend daily and waited one day for him to attack her to kill him, would this truly be considered self defense?

Even more controversially, if a man attacks an armed soldier who is entering his home without a warrant or due notice, is he a killer? If citizens of a nation attack an occupying force, are they terrorists or nationalists? Is there a difference?

What are your thoughts?