That’s a very complex question with many, many answers. No individual life can be boiled down to a single phenomenon. A lot of the answers I’m seeing in here are great, ans definitely describe a phenomenon at play, but it’s important to remember that nobody’s just outright stupid enough to fall for a single piece of rhetoric. Instead, them coming into bigotry is the result of a complex web of ideas that brought them to that conclusion.

That being said, I’ll add my two cents that I don’t see anyone saying: privilege. Privilege insulates people from how cold and cruel the world can be; in doing so, they don’t learn the comraderie that grows out of shared hardship (aka empathy). They see others experiencing it, and assume they are weak, both for “allowing themselves” to fall into hardship, as well as for “getting conned” by others who have fallen on hardship. This too adds fuel to the fire that is all the other reasons people get pulled into hateful ideologies.

Imagine being excluded from some perceived secret club based on conditions you didn’t have a choice in, and seeing women or bipoc or lgbt or the working class supporting each other. You too would feel resentment towards those who won’t include you in their circles. Yet you never developed the proper understanding of the ties that bind them, so you only see it as hate towards you and your demographic; this then becomes a feedback loop: your hate hurts thode communities, making them even more interdependent on each other, making you more resentful and frustrated.

You fall in with people you don’t really like because of a shared disdain for The Others, and then, because that’s your only lived experience, assume all identity-based comraderie is necessarily just a loose collective of people that only get along because of a common enemy. This reinforces your belief that The Others hate you, only adding fuel to the fire of your own hate.

This is also why these people are so easily manipulated: all you have to do is control their perception of who hates them, and they’ll do whatever you say to make it stop. This is why politics and religion are such great examples, and no “side” is immune. Want to make a leftist out of a fascist? Convince them that The Jews are actually just the bourgeoisie, who must be killed for the good of ourselves and our nation. An anarchist who fears authoritarians will readily agree to being a part of an exclusive coalition of individuals that determines the way society is structured, so, y’know, the authoritarians don’t get their way.