As Thomas Sowell has said: 'I have never understood why it is greed to want to keep the money you have earned but not greed to want to take somebody else's money.'

Because capitalists do not earn their money. Their laborers do the lion's share of the work in a cooperative nature. Many key inventions occurred due to government subsidies that came out of the taxpayers' money, meaning it did not rightfully belong to the tiny few who profited off of it. Profiting off of other people's work and money is something capitalists do as a matter of course, and eliminating the government that helps make this happen, and routinely bails out the capitalists when they periodically come close to destroying the economy by using other people's money on high risk speculative ventures.

It's not greed to want to have security and a lot of nice things. No (former) capitalist would lose that security and everything they have if a true Marxian socialist system was established. What they would lose, however, is their power and privilege, and that is what they are chiefly fighting to preserve. The only thing taken from them would be the industries and services that rightfully belong to the people who operate them and do the work to make the abundance that only a tiny handful of people actually get to enjoy anywhere near in full.

Remember: The most serious form of greed is when what you want to keep deprives a great majority of the same type of life you have.

We can see how nations that became wealthy thanks to free-market capitalism

Only a tiny handful of the population in these nations had access to the vast bulk of that wealth. One of the problems of capitalist supporters and pundits is that they base wealth and success of a nation on what that tiny handful has, not what the vast majority has. Capitalism did indeed assist the Industrial Revolution into coming about, but now that we have these advances, it's outlived its usefulness to the world, and it's time to move onto the next advanced form of economic order... just as we did in the American Revolution, when the new nation dispensed with the previous economic world order that had similarly outlived its usefulness--feudalism. Capitalism was a progressive system at the time. Not any more. Just as feudalism eventually made capitalism possible, so capitalism eventually made Marxian socialism possible. It's time to advance to the next stage, despite how reluctant so many people have always been to do that historically. People tend to prefer what they are used to, even if what they are used to is not a good thing.

later stalled when they adopted socialist ideas (most notably several Scandinavian nations) before dropping those ideas;

Sorry, these were social democratic ideals--i.e., liberal capitalism--not genuine socialism. I'll correct this dishonesty of yours every time I have to, sorry (well not really!). Nevertheless, these liberal (not socialist) social programs gave these people a much higher standard of living than they did here, including things like universal health care. You won't see libertarians talking about that. See, this is how libertarians and call capitalist supporters define progress and success: those which the tiny owning handful achieve. Any controls on what they can bilk from the vast majority who do all the work is considered a set back on all society.

we can see how Venezuela slipped from the wealthiest nation in South America to the poorest - with skyrocketing poverty and violence as the people cannot find enough food to eat despite having some of the largest oil reserves in the world,abundant natural resources, good soil and a year-round growing season;

I explained elsewhere in this thread how 1) Venezuela was never socialist, but attempted a social democratic system of liberal capitalism; 2) How they lacked the technology to take full advantage of its natural resources. And are you seriously saying that totally unregulated capitalism, with no social safety net and a price tag on just about everything, would enable these people to rise out of poverty, rampant crime, and the brutal competition that causes the violence you mention? Since when has unregulated capitalism ever allowed anything resembling prosperity for the vast majority? A class-divided system is not designed to do that. You are spewing propaganda that pretends to care about the average person when in actuality it actually supports no encumbrances whatsoever to the tiny minority who actually controls all of that wealth.

we can see how Chile which was once one of the poorest nations in South America became the wealthiest after partially adopting free market ideas.

And the general public outside of the few who control the corporations that take the lion's share of that wealth are certainly doing well! But then again, that's not how the success of a nation is determined by the capitalist method.

We have seen the same pattern over and over again, everywhere we look, and some of us have come to suspect a causal link.

Yep. Capitalism and class divisions in general. And, yes, money, lest I forget.

But for some reason, some people are not convinced.

Indeed. Because they do not want to give up on what they are used to, and cannot get past the fetishism for money and power, and the kingly status of the few who benefit from it, that has been instilled in our cultural framework by the people who write the textbooks. Btw, those people are most often employees of the state, who has long supported the hegemony of the very class who hates them for daring to put limits on what that class can take from the vast majority and the environment (these limits placed there to save the system, not destroy it, by giving it a modicum of what humans call compassion).

They prefer to accuse others of greed for wanting to keep what they have produced,

Only those few who own do not produce. Those who work produce, and they are the vast majority. And it's human nature not to want to live in comparative poverty when an abundance can be provided for all. A few people wanting to hog a full 80% of that abundance for just themselves is what true greed is made of.

even as they demand the product of other people's labor for themselves.

We should all benefit from the collective labor of each other, and everyone enjoy the full fruit of it without the lion's share going to a tiny minority who "own" rather than produce it. Do not mistake ownership for "producing." When I write a story, I produce it, not the publishing company who publishes it. They just provide the money, not the labor. Let us not forget that.