While scapegoats come in handy for placing blame, there's unfortunately no such alternative for guilt (because Cobb could really use a guiltgoat…or whatever you would call it).
Cobb is dealing with a phenomenal amount of guilt. In his mind—and it's his mind that sets forth a lot of the action in Inception—he effectively killed his wife and brought a parentless world upon his children. Gulp.
Questions About Guilt and Blame
Should the team feel guilty for incepting Fischer? Is what they're doing ethical, or is the way in which they're manipulating him immoral despite the outcome?
Could Mal possibly be a positive influence on Cobb?
Does Ariadne help Cobb realize he is not at fault, or does she make him realize that, although he is guilty, the guilt must be overcome?
Chew on This
Cobb's guilt is brought on by grief. He doesn't know how to deal with the loss of his wife and so he has decided to place the blame on himself. It's easier to hate himself than to admit that her death was out of his control.
Cobb's grief is brought on by guilt. He knows he was responsible for his wife's death. Mal will definitely be returning to him until Cobb stops running away and turns himself in.