You have to remember though that, like Arthas, Deathwing used to be a good guy. And like Arthas, insanity overtook him. Unlike Arthas, Deathwing has had thousands of years of insanity. He got epically pwned during the Second War, which necessitated him getting metal plates grafted to his body to keep him from imploding.

You'd be mildly pissed too. ;)