That mother hears the voice of God, and He says to kill her son.
In this game, a boy named Isaac is being raised by a fundamentalist Christian mother.
It was a hit, and not long after, it was remade with new content and graphics, as The Binding of Isaac: Rebirth. He shared beautiful, weird games on the website, and one of the last ones he posted was a demo for a new game, The Binding of Isaac. Back when I frequented, one of the most well-known game developers on the website was a man named Edmund McMillen.