Ah yeah, wealth inequality. E.g. lots of rich bastards own megayachts and I don't, therefore this justifies me stealing packages from other common people who also don't own megayachts.

These people aren't Robin Hood, stop excusing their criminal behavior.

I didn't excuse their behaviour.

If you actually care about reducing crime you look at the evidence. E.g. https://equalitytrust.org.uk/crime

So, are people like animals that start going feral when they see someone in a way better status than they are? If that's the case, and this article suggests there is psychological evidence to support it, it poses a serious problem for the whole society. Are we really going to assign a maximum value to people's wealth?

People can reasonably be excused for theft when they are trying to meet their needs. Somebody who steals because they're at the poverty line and are trying to keep a roof over their head and food on their children's plate deserves sympathy.

But when people have their needs met, are above the poverty line with a decent job that pays the bills, and they still steal because "inequality" makes them jealous and greedy, they don't deserve sympathy. Most people in such a condition don't become theives, they either content themselves with what they have or they try to better their condition through some honest means, like seeking better job training or political advocacy for social change. The ones who respond by turning to crime (usually targetting other common people living in similar conditions as themselves) are antisocials who need to have the book thrown at them. The reason they're stealing packages instead of robbing banks or spending their time looking for a better job is because porch theft is easy and low risk. If penalties and enforcement were increased, theft rates would drop.

Why would inequality be the issue?

If 1% of the population suddenly got personal space-ships (hugely increasing inequality) would that make people steal from trains more?

It's an issue because humans are humans. E.g. "The link between economic inequality and both property crime and violent crime is well established" https://equalitytrust.org.uk/crime

A link there may be, but that can be A causing B, B causing A or both A and B having a common cause.

That's a nice theory, except most people don't turn petty thief just because billionaires exist. These thieves aren't class warfare warriors, no matter how much you wish it. They're greedy degenerates who don't care about others. Being an asshole isn't the exclusive domain of the rich.