After Adam's sin, what previously he could have attained through performing a single mitzvah, would now take many mitzvos to accomplish. If one were to take all the souls and put them together, they would equal Adam before he sinned. If Adam hadn't sinned, then he would have been the Mashiach. Before the sin, the evil inclination or the "other side" was outside of him. Once he sinned, the yetzer hara became part of his consciousness and all of creation and the world is said to be in a state of irbuvya, where good and evil are mixed together. As each generation sins more and more, people retain less free will. Death comes to repair the corruption, after which the soul would be allowed to go to the World to Come.