Your claim is that when people believe in karma it becomes real (through guilt). Karma is justice, it's a claim that justice is natural and will occur without human intervention - and also as an explanatory tool for why some people suffer and others are lucky. Without karma, you have to use human systems of justice because you can't rely on justice happening naturally. When I say justice, I'm talking about the process by which fairness is maintained through punishment or other resolution of harms done.