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I don't think that's the whole story. I read a really eye-opening book a while back that was essentially an insider's guide to surviving in prison, and one of the things that stuck with me was the idea that people behave themselves (for the most part) in that environment because there's always somewhere "further down" in the system they can send you as punishment. It's the same on the outside. In the US, at least, you have to be really, really poor before being in prison for the rest of your life is a better option. Most poor people have lives, friends, family, a daily routine, freedoms, possessions, etc., that they'd be putting at risk if they decided to start murdering random rich people. Especially at scale.

I really want a citation for that.

That is not true. That standard has historically applied only to people who are "in your group". People throughout history have had no problem killing a neighboring tribe and taking over their stuff (including women and children).

I doubt any poor person sees a rich person as a part of their extended social group. The psychological barrier does not exist.