Usually one of the first steps is dehumanization - make your targets "less than human" in the eyes of the population. Nazis famously did it by comparing Jews to rats. You'll notice in a lot of recent Israeli press releases and media Palestinians are referred to as "inhuman animals" or some variation of that. By creating that disconnect between your targets and "normal, healthy" humans you reduce empathy and make harsher treatment seem acceptable.

Another step is to make your audience disgusted or angry. Studies show there's a link between those two emotions and harsher judgments (although degree/method is still very much an area of research). To invoke disgust you may use words like "filthy, wretched, diseased, mindless" etc. Using the Nazi example again, they made cartoons that showed Jews as dirty, greasy and generally disgusting. To make people angry convince them your targets are "immoral, violent, bloodthirsty" and so on. Nazis leaned heavily into blaming Jews for society's ills and calling them thieves. Both effects can be made greater if your audience is conditioned to be sensitive to anger/disgust, i.e. being raised to believe in strict definitions of purity and so on. For Nazis it was the idea that Aryans were racially superior. For Zionism it often involves teaching people they are "God's chosen" with other races not having the same rights (like rights to dwell in territory claimed by Israel) because of religion.

So if you can make your victims seem less than human and enrage or disgust your audience you convince people to do horrible things. They won't feel like they are doing it to valuable humans and often think it's a form of justice or necessary cleansing. Using the above psychological "levers" can shift perceptions so large populations view some other group as different to relatable, valid people.