One, he's been the king of heaven before and it ended in catastrophe and blood.
Two, "I have learned a new way! I will not strike you back! I will turn the other cheek, unless it seems like I am actually in danger, and then I will kill you."
Jesus turned the other cheek until they bad guys killed him and they felt like dicks.
Doctor Who gives you one chance.
Castiel is not employing new methods, he is just still killing anyone who disagrees with him, and he's such a ponce that he believes that it's completely different that he's now only killing people who threaten him.
Or is that different?
(The final season is when they rip off Preacher and go find God.)
Then they get renewed and God has to be a dick who is taught a lesson too.