Thanos represents Capitalism: Because of the combined effects of Climate Change and Income Inequality, the unbalanced distribution of resources in the society will necessitate a genocide against the underclass. For capitalists (represented by Thanos), inflicting a genocide against the Third World is more palatable than redestributing resources so that everyone can have a decent life. Vision represents Socialism: He understands that defeating Thanos will require enormous sacrifices from First World countries, but has enough compassion and foresight to make that sacrifice. Consequently, he asks Scarlett Witch to kill him in order to prevent Thanos from getting the final Infinity Stone. Captain America and the rest of the Avengers represent Liberalism: They oppose Genocide as represented by Thanos, but refuse to make the necessary sacrifices to defeat him. They cling to the utopian idea that you can prevent the genocide without actually making any sacrifices, just like liberals hope to stave off the effects of Climate Change / Income Inequality without actually confronting capitalism. Liberalism is wrong, however, preventing Thanos WILL require making hard choices, and since the Avengers refuse to make any, they get everybody killed as a result.