I appreciate your commitment to civility. That's rare here. :)
Now, to business. The core difference between capitalism and socialism is that capitalism utilizes human greed and runs off market-based interactions between private property owners. That reliance on greed is its strength; if someone want something that someone else has, and there is a market to facilitate legal trade, then they will negotiate a mutually beneficial deal.
Socialism on the other hand disavows, either to a large degree or entirely, markets. Instead it emphasizes that 'everyone owns everything', and that the common good should be the goal of everyone. Directing the work of the populace is an entity of some sort, ensuring that whatever society needs is being made. This is the ideal system, because it would mean everyone is being selfless and helping others, but we know that most people are not selfless. Like I said, it is inevitable that pure socialism becomes a dictatorship. So yes, you are correct that human nature, and not the theoretical system, is the problem. However, something working only in theory is a rather large problem that we cannot surmount on any practical level, and therefore is not worth trying to defend (IMO).
Now, you noted that we utilize aspects of socialism today, and that unchecked capitalism can also become tyranny (presumably through monopolies). There again you are correct. We are not a purely capitalistic laissez-faire (Free hand) economy, but a mixed one with free capitalistic markets and socialistic education and welfare programs (welfare is, after all, a redistribution of formerly private property in the form of money). This is not inherently a bad thing. In fact, I think our fusion of free markets and private property with basic social programs is a good thing and a large part of why America is the best country on earth.
My primary argument against socialism, the fact that it can't work as a national system because of human nature, has been beaten to death at this point. I'll leave you with these thoughts and see what you think.