One Question: Why was Cobb's projection of Mal evil? Why would his subconcious project her as an enemy?

In my opinion, Cobb's project of Mal is symbolic of his guilt over being to one who planted the idea in her head that drove her to kill herself. Cobb's projection of Mal is "evil" because he feels that she has the right to be angry with him for planting that idea and damaging her life, his life and the lives of their children. Essentially, Cobb is punishing himself kwa having his projection of Mal punish him.

But thats just my interpretation.