Why did Mr. Cobb blame himself for his wife's death?

He blames himself because she didnt want to leave limbo so thats where he took the tuktok (which was originally Mahls) and left it spinning in her childhood tahanan that Mahl re created in limbo. Cobb then told her that if the tuktok kept spinning that she was in a dream and she needed to kill herself to wake up.he did this so that when she saw the tuktok in her child hood tahanan that she would go back to reality with him. But when they went back to reality she still thought she was stil dreaming and kept trying to kill herself. And she eventually succeeded. He blames himself because he put the idea in her head first.

He planted the idea that her life wasn't real in her mind so that she thought killing herself was the only way to escape. Basically, he made her go insane.

He blamed himself for Mal's death because he planted the idea in her mind that her world was not real. He did this when they were trapped in limbo, and didn't know that it would carry over to when the woke in the real world. This is how he knew Inception was possible.