It's because they don't see them as people, they see them as violent criminals that the world would be better off without.
If you step into their shoes for a minute, and one of the criminals you just successfully took off of the streets said they can't breathe, your first thought might be "good. Maybe that'll teach you a lesson about doing crimes in my neighborhood." Your second might be "I wish I could shoot you right now and get this over with, but maybe I'll get lucky and I can say I didn't hear you."
Note that the second one is inherently a stupid thought, there's body cams. That kind of logic didn't stop my 5-year-old from telling me she cleaned her room when I could easily check and find out she didn't, and it won't stop cops from fantasizing about everything working out here.
That's exactly why they do things that way. They're living out a fantasy world where there are no real rules and there are no consequences, and they have to live a balancing act between indulging in that and dealing with reality. Sometimes cops fail to balance that, and that's what we see here.
As for who trains them, it's their fellow cops. This isn't a bunch of individual fantasies, these men work and train and talk together about how it'd be so much better if they had less restrictions and just talk about that hypothetical world. New cops who have any kind of racism or similar "My group is best" can join the conversation and add in their own unique version to the group fantasy. New cops who aren't already racist, though, won't hear blatant racism. No, they will just hear about crime stats and reoffending rates, about cops that died trying to deal with all the supposed crime, and about how stopping them is justice and will help everyone, not just cops. In time they'll share the group fantasy, too, and stop seeing their victims as people. Occasionally someone just doesn't join in the fantasy and they get bullied until they quit.
This is why the easiest way to move forward from this kind of thing is to gut the police departments and start over, or we at least need bodycams that can't be turned off so easily.