Oh, easily. "Othering," "God allows it/wills it," etc. I'm not trying to do whataboutism here, but reading the excerpt you provided, I was struck how similar it was to accounts of the Rape of Nanking. Heck, it's easy to compare to what's gone on in the last decade or so - there's recently been a soldier's account from Myanmar, of orders being given to soldiers to basically "go in and shoot everyone" (everyone being the Rohinga).
None of this is to sanitize the conquistidors, but it's very easy to get humans to kill other humans.
Which is kind of a mark against pacifist civilizations if you take the Darwinian point of view. To borrow a phrase from Doctor Who, "show me a peaceful species, and I'll show you lunch."
Even if anarcho communism exists with agriculture, it's why I'm dubious that it could ever work. If every group on Earth decided to "go back to the land," laying down their arms and whatnot, then all it would take is one group to not lay down their arms, and declare the communists' territory free real estate. Pacifism's great in theory, not so great in practice.