I am trying to figure out why institutionalised mindless violence is acceptable. And why apparently rational people take up guns and invade foreign countries to justify their ideologically mindless murders.
You are injecting your preconceptions and final conclusions into the question, so it is clear to me that this is just a rhetorical device.
Of course you will never have an answer to these questions. Human behavior is not significantly different than it has been for tens of thousands of years; why do you think it would change?
Humans are fear-filled primitive savages - overall, a cursedly ineffective species. Humans have just enough intelligence to make a few clever tools, that's all. Give it a few more millenia; natural selection will squelch the human behavior that is disturbing you. Human beings will be a barely discernable blip on the chart of failed species.