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.

When I was bullied starting in kindergarten for being "weird" (aka undiagnosed autism) where did those kids learn to be so mean and cruel? They were too young to be exposed to all the propaganda and this was before home internet was common.

I was bullied continuously from some of my earliest memories until I graduated high school, and I have a deeply internalized sense of being a subhuman freak that most people look down on that has never left me, and I personally cannot imagine it ever leaving me, though I must acknowledge that it's possible that some day it will. I was subjected to violence across years for a variety of absurd reasons, so even if we continue to disagree, I'd like to emphasize that this isn't an idle subject to me.

I want to really strongly encourage you to just spend a little bit longer genuinely thinking about the question you posed, because the conclusion that you draw is not actually self-evident from the question. Did kids call you "weird?" Do you suppose that they invented the word and it just happens to line up with how the rest of society uses it? Or that after hundreds of thousands of years we've just had this monolithic conception of the understanding of what constitutes "weird" and how to respond to it that is baked into our DNA and hasn't changed this whole time? No, obviously whatever we can remark about natural human propensities, there are still many explanations that are dependent on social conditioning, because people begin that process as soon as they are born, and the environment that their parents created for them/exposed them to is something that had already had years of time to shape them during the most impressionable period of their lives.

My personal opinion is that our cultural promotion of shame and punishment, our moralizing about people being inadequate and that people can deserve to be made to suffer by those with power, is fundamental in understanding how young children interact with each other and breeds a very early form of false consciousness. By making you the object of punishment, they can protect themselves, and they can draw a line where they are the punisher and not the punished, the shamer and not the shamed, by delineating traits that put them with the vast majority and you in the minority, picked out because you're in some manner vulnerable (as we would expect a little kid with undiagnosed autism to be in a social situation). They are more powerful than you and can assert this by making you suffer, just like their parents taught them by doing that to them. Do you suppose this behavior would be just the same if their parents did not teach them shame and punishment as fundamental mechanisms of how they experience the world?

Mind you, I'm not saying that if we imagine kids raised without these and other reactionary norms, that they would not be able to reinvent cruelty themselves (obviously they would be able to), but I think it's a misunderstanding to treat cruelty as a fundamental principle of "human nature" rather than something that people end up choosing as a result of circumstance, just as people also choose kindness as a result of circumstance (there have been many studies about little kids trying to help people who seem to be in pain, for instance). It is arguably the most characteristic trait of intelligent life that there is a high degree of variation in individuals, in large part due to differences in experience even within the same small group, and there are likewise differences you can observe in different groups, different "cultures" and so on. We are so heavily the product of our conditions that it's very difficult to identify fundamental traits, especially by looking at narrow population slices. Because of that, when we want to answer any question about why some group of humans are one way or another, it is critical that we seriously examine their conditions to try to understand what role those conditions play. Otherwise, we end up falling into unscientific essentialism like "race scientists" and the like.