"Why I do trifle thus with his despair
Is done to cure it..."
See in text (Act IV - Scene VI)
Overtly, this line means that Edgar is playing with his father in order to cure him of his despair. He will get his father to jump off of what he believes is a tall cliff and realize that he does not want to die when he jumps onto flat ground. However, the syntax of this line offers another reading: Why do I fool him thus when his despair, or desire to kill himself, is enough to cure him of it?