The apprehension people feel about killing others is a fairly universal thing that I don't believe to a purely social construct like "morals" (whatever that is).
I am fairly comfortable labeling "doesn't mind killing people" as sufficiently abnormal to be called "ill".
You're not correctly quoting the parent comment (I'm not sure you're quoting anyone, it looks like you're erecting a strawman), he said "willing to kill".
There is a lot of ground between being willing to kill, for example to save the lives of others, and not minding killing.
The apprehension people feel about killing others is a fairly universal thing that I don't believe to a purely social construct like "morals" (whatever that is).
Well, once you've thrown away morals, it's true that all you have left is the hope that others will refrain from killing others due to not feeling like it.
Morality is just the portion of goals that humans in a given culture share, but I'm not sure you should be so quick to dismiss it as a positive force.
I think what we call "morality" probably has a biological basis as well. Humans are social animals. Hence, we're going to have selection pressures towards behaviors (and the internal thinking required to produce those behaviors) that preserve social groups. If we didn't, human groups wouldn't survive, and we wouldn't be social animals. I think we can point to other social animals as examples. There's also support in the fact that there are broad similarities across cultures about what is moral - for example, murder is wrong. If there was no biological basis, I think we'd see far more variation in what we call "morals."
That's a peculiar definition of "sanity" (not that there's an objective way to define it anyway). Humans have been fighting wars since ever, just like other animals in herds. There's maybe an evolutionary advantage to it. Nations create armies to protect themselves and assert their territory (ok, nowadays it's interests). Since "sanity" is a socially defined construct, there's nothing wrong with the soldiers.
I think you have underestimated how much culture determine people's attitudes.
Consider a normal guy from my home country a few hundreds of years ago. He believed that he will only go to Heaven if he dies well in battle and is hence really scared of dying from disease. He wasn't a psychopath; he loved people and would die for his friends etc. He is normal for a clan society.
The same genes sit in me, which sincerely hopes never to have to maim or kill anyone in my life. Those human genes program us to be very programmable with the present culture. A powerful tool can often be misused.
The trick is to make the culture stay "nice". And to not have neighbours from clan cultures.
Humans have been fighting, yes. Fighting things humans understand, and for traditional human reasons, at their discretion.
What's new-ish (5-10k years) and completely against individual success, is a large and largely unwilling army driven to kill for reasons they don't understand or are lied to about.
If you are looking for a mass delusion we all participate in but understand little, look no further than the money in my pocket. OT, this article has some interesting insights on how organized states (and armies) actually led to less violence historically: http://edge.org/conversation/mc2011-history-violence-pinker
My problem with armies is that we have these essentially slave-cults for protection and we call ourselves free. Better off, sure. But not really free because we've got this slave system (Military justice/prisons) to enforce discipline in our enslaved warriors (they might have chosen to serve, but you can't reasonably give consent for personality breaking) without whom we're unsafe - or so goes the theory.
I simply feel that each and every one of us needs to tote a gun through mud, and know why we're willing to do it, to live in a society that practices war. Not only so that we choose wisely, but so that we're really safe - not just under guard.
How is such a thing logically possible?