Come to think of it, if Wardens were allowed to kill Warlocks with magic, that might explain the sort of fanaticism we see from Morgan. Remember that the root of the First Law is that when you use magic for something, you believe that usage was right and correct. So if, say, Morgan uses magic to kill a Warlock, he believes more and more that killing Warlocks is correct -- and moreover, that he is right and righteous in deciding who is a Warlock and who isn't, and thus who deserves to die.
I mean, take the word "warlock" out of that last sentence and it's word-for-word the justification for the First Law's existence.
I disagree with this because Morgan was fanatical about upholding the laws of magic. Even though he believed with every fiber of his being that Harry was guilty, he still did not kill him. I'd say Morgan was probably just about as strong in his belief about upholding the laws as Michael was in his Faith.
Also when killing with magic it isn't just your belief that makes it easier. The wizard is twisting the creative forces of the universe, and that abomination of creation goes through the wizards being. Harry used his magic in self defense, but in Storm Front he nearly gave in to his desires to kill, and not in self defense, but for power.
"I prepared to destroy the lake house, the Shadowman, and any of the pathetic underlings he had with him. With such power, I could cast my defiance at the Council itself, the gathering of white-bearded old fools without foresight, without imagination, without vision. The Council, and that pathetic watchdog, Morgan, had no idea of the true depths of my strength. The energy was all there, gleeful within my anger, ready to reach out and reduce to ashes all that I hated and feared."
Now this could have been from using his Sight on the home of Sells, I don't know. He didn't mention it causing his feelings.
Come to think of it that gives me an idea... new thread!