The road to the hell of human cruelty is one of letting slip one bit of decency to cruelty one small step at a time. Our lower natures are pricked by fears stoked, and fear transforms to anger that is directed towards an "other", someone not of my tribe or culture or ideology. A person with a name and face becomes a symbol, and when I fail to see that person but only a symbol to be crushed then we slide down the road towards our worst potentials. This is what tyrants use as their stock and trade to power. They learn how to pull the triggers, to stoke the fears, to create the anger, to focus it on scapegoats - who just happen to be people like you and I. This is the dark art of propaganda, the big lie, the dehumanizing judgment that leads to the horrors.
So how do we avoid this?
I do it by recognizing when my triggers are being pulled, and especially those triggers which create anger that transforms to hate and indifference to the suffering of others. We can inoculate ourselves against it through seeing each person as fundamentally equal and worthy of the same good treatment that I would expect for myself and my loved ones. I have to always see the person, to not a reduce a person to a symbol, to someone to be feared on account of differences that are not in fact dangerous.
Be aware of the man or woman who stands on a soapbox who stirs and shapes the emotions of people into fear, anger, hate and indifference. Beware of symbols on banners becoming standards to lead people as an army towards the glory of battle against ________. Watch the faces and bodies, to see how they transform in response to the leader and rhetoric being espoused. We are instinctively and emotionally driven far more than we are intellectually driven. That has a positive and a negative aspect. When it is sympathies that are elicited then it humanizes, and when it is antipathies that are elicited it dehumanizes.
I let slip my decency to cruelty one small step at a time.