you’re correct: we’re not ‘supposed’ to take souvenirs, but it’s been done throughout history, and before. it’s a behavior that goes back to the very beginnings of humanity.
it’s an urge that lies beneath our ‘rational’ natures, just below the surface. under the right circumstances, it comes out, as it did in this case.
there are other behaviors, equally or more repulsive, that could have expressed themselves as well.
conflict brings out the most primal portion of ourselves. if order is not strictly maintained at all levels of command, it can very easily express itself into what we now call ‘war crimes’ or ‘atrocities’.
behavior of any type is far, far harder to change than technology.