If you tell one lie, are you a liar? I should think not. A liar is one who habitually lies. Otherwise, we would all be liars and the term 'liar' would perish from lack of contrast.
If you have been seriously drunk a time or two, are you a drunkard? I should think not. A drunkard is one who habitually gets drunk. Otherwise we would damn near all be drunkards, and the term 'drunkard' would perish from lack of contrast.
This rumination is iterable across thief, lecher, glutton and other terms of moral disapprobation.
But if a man commits murder just one time, we call him a murderer and we feel justified in so doing. We would find it ridiculous were he to complain, "I shot man in Reno just to watch him die, but I am no murderer; a murderer is one who regularly and habitually does the deed."
How about rape? Does one rape a rapist make? I think we would say yes.
So what is the difference between murder and rape and the other cases? The gravity of the crimes would seem to be one factor and the relative rarity another.
More grist for the mill.
It is not easy to think clearly and deeply about moral questions. Few even try.