If a big war started would there be big atrocities again like in history?

I live in Australia so I worry about china a lot, and I worry because we have strict laws about treatment of POW and civilians and stuff but countries like China and Russia and north Korea probably would torture and kill innocent people :(

and my friend showed me a video of some Russian guy cutting another person's head off and I've heard my parents talking about Azerbaijan doing bad stuff to Armenians

So clearly that kind of bad treatment still happens today, so if a big war started then would there be anything to stop massacre and genocide?