I'm not talking about whether or not people enjoy the act of taking another person's life. Obviously, some people do. I am talking about bragging about it. If you ever hear someone making these kinds of claims loudly at a bar, you can bet your next paycheck they never left the FOB.
I don't doubt that betting against it is a good bet. But I do doubt there is zero intersection between braggart and people who enjoy killing.
Perhaps you would accept essentially zero in that case.
I think it's a small minority, but probably not as rare as you make it out to be: https://www.quora.com/What-percentage-of-people-are-psychopa...
I don't mean any offense by this, I'm trying to say it delicately, but I think "anybody who brags about killing in combat is lying" is something of 'white lie', probably earnestly believed by most who repeat it, meant to preserve the reputation of combat veterans as a whole by discrediting any veteran who would be so tactless as to brag about killing. I believe it's basically a way of credibly stating "this person who's making the rest of us look bad by association does not represent us."
Good luck in your quest to nail down precise numbers. You'll never get there, but I wish you luck in any case.
I strongly suspect there are exceptions to this rule. At the very least we know there exist people in prisons today who enjoy murder. Arguably that's very different since (I assume) 'enjoying killing' and 'enjoying getting shot back at' are worlds apart. But, would it surprise you if there was a small minority of drone operators who genuinely enjoyed their work?
Then again I suppose (hope) the modern military system might select against that kind of mindset.