where did those kids learn to be so mean and cruel?

The same place they learned any of their social interactions: the other humans they interacted with or witnessed interacting. Obviously that means their parents first but also the community they are in, from extended family to neighbors to anyone they see their parents interact with. And in our modern era, it's not limited to their family and community, but literally social conditioning from the media they're exposed to. Even toddlers will reenact what they see on a television or computer screen. I don't remember all the age milestones of where children become capable or aware of what, but they can mimic the behavior of others they see from a very young age, earlier than kindergarten.

They were too young to be exposed to all the propaganda

No they weren't. Why would you think this?

I've learned from many years of being around these people that many of them are just hateful disgusting leeches at the end of the day I truly believe our species has this baked in [selfish and greedy]

That is not a materialist view of reality. You save it by saying

capitalism encourages that behavior.

Which is the key, but which you seem to just say in passing rather than recognizing it as the counter to the rest of what you were saying.

Greed and selfishness may be "baked in" to humanity, but no more so (and arguably much less so) than empathy, compassion and social altruism, no more so than kindness and a very human desire to be helpful to others. What is truly "baked in" to humanity? Adaptability. Humans are highly adaptable to their material conditions (technically, all life is highly adaptable to its material conditions but humans are also highly adaptable to the emergent property of their social conditions). An evil and cruel system based on a capitalist mode of production is going to produce and reproduce a sick society which is going to produce and reproduce sick and twisted people. That's what class struggle ultimately aims to rectify.