"Your penance maybe." Michael said after taking a breath and opening his eyes again. He could keep control, he would put a lid on his pain and rage, containing it until he had a chance to put it to use. Bringing it all down on Crowley right then would have been a waste of time, and if Michael was completely honest with himself, he wasn't particularly interested in destroying one of the few hands that had been offered to him in friendship. He could excuse the demon's involvement for the moment, he was only behaving in accordance with his nature, Raphael and Castiel should have known better.
He finally looked to Crowley again, "It seems I am foolish as well. I made them the promise that I would stay away. I let them know I had concerns and problems greater and unrelating to them. But the more I hear about Castiel and what he has done, well, we may no longer be in heaven but he was still created to be a soldier. That makes him doubly mine. Mine to punish and correct when I must. That means I am obliged to break my promise."
That had the potential to blow up in his face if he took it too far. Michael knew that and would do his best to be careful.
"Not yet though." He nodded to himself. "I need more than the word of a demon, as I am sure you can understand. I rather not act before I had all the information." And he wasn't sure he was strong enough if Castiel decided to fight against him. Seraphim could be formidable, Michael himself had made sure of that in the early days when he had the time to be much more hands-on with the garrisons.
"Also, I would appreciate it that when the time comes, for the sake of this growing tolerance between us, that you do not get yourself involved. I have no desire to kill my brother, I don't believe I have the heart to do that, as humanity would say." He truly didn't intend to kill Castiel for it, that would have been too much and might have resulted in the flannel brothers finding a way to shove him back into the Cage. Punishment was needed though, possibly more by him than Castiel.