All of the above is as good an answer as any in this case.

My vote would go to the mass framing moral decisions as immutable human nature.

What would be the benefit of greed if there's no money or barter system to incentivize amassing more than you could use yourself?

Evil is a human concept that we define. If you want to, you can extend it to explain non-human animal behaviour. I actually wouldn't, but then I also don't believe evil is real. It's just a way of saying something is bad without being specific as to the whys. Generally, we are able to be specific to the whys so "evil" is mostly appropriate for comic book villains and the like. Satan is evil. He also doesn't exist.

Humans are instinctively selfish, just like other animals. If a serial killer kills for sexual gratification, that for me is an example of extreme selfishness. If you keep eating meat despite a conviction that the world would be a slightly better place If you became a vegetarian, then that, to me, is also an example of selfishness. Being stingy about the price of clothes when you know textile workers suffer is selfish.

Democracies work for people because they let most people get what they want in a way that seems reasonably fair. But it also gives our selfishness a lot of freedom, f ex. to make choices that are good for our own comfort, but that will add to the suffering of other people or future generations.

Well, sure you could debate the "nature of evil" or even whether it exists till the bovines find their way back to where they originally set off from, but that's not really the question. I framed this poll in a way that asks what do you consider to be the root of what is generally accepted as evil, and I gave examples. Sure, you could say (I disagree) that being a meat-eater is selfish - I like my meat and I HATE vegetables - but I didn't ask what's the root of selfishness? In the same way, not all evil is necessarily selfish. That's another question altogether. Selfish in one way, I guess, as you are satisfying your own needs - be they sexual, racial, territorial or financial - without considering those of others, but if I had time to think about it (about to make dinner so quick post) I would imagine there are examples of evil that are not driven by selfishness. So some of those examples you give would not, for me (and I think for most people) fall under the heading of evil as we understand it. I know you're changing the question to what is selfish, but I don't think that really helps, or applies here.

I was assuming there would still be some kind of barter system. I think as long as material possessions exist, so will the amassment of them for the sake of bartering

yeah you can do evil without necessarily just doing it for what we would call purely selfish reasons in terms of status etc. Someone can be motivated by an ideology to do evil and be so dedicated to it they will actually make sacrifices for it. Like Osama didn't need to turn to extremism, he had a pampered life in Saudi Arabia and ended up living out of caves in Afghanistan.

Obviously whether what he did was even evil is a matter of interpretation. But for whatever category of behavior you categorize as evil there's gonna be a variety of potential sources and not one singular root of all evil.

If you do "evil" for non-selfish reasons, then someone will probably think you're doing good. It becomes a matter of interpretation. Some see a terrorist. Others may see a holy warrior fighting a great evil.

People thought the Nazis were good too. Why is it that if a different culture or whatever thinks it's good that's not evil but if it's just one individual psycho who thinks it's good then it is?

That's fair, but I think the blame falls more on systems that reinforces exploitation.