Michael manages to look high and mighty even from a lower seat. He's doing it right now, cool and vaguely annoyed, like Lucifer's an underling wasting his time with a minor complaint.

He knows at least in part what happened between Lucifer and Gabriel. Given Raphael's access to the prophet, there's very little that happened on Earth related to the Apocalypse that didn't eventually make its way to his ears. Funny enough, it's one of few incidents that didn't further lower his opinion of Lucifer. It might be another story if it had featured an angel in the service of Heaven, but for all that Gabriel is dearer to him than any lower angel, he abandoned them. He interfered in a matter that he'd long ago recused himself from. What else did he expect would happen?

"Trying to kill someone does not mean you want them dead."

Whether or not Lucifer likes that truth or wants to recognize it, he must at least understand it now that he's had a turn in Michael's shoes. Gabriel might see it too, though whether or not he's willing to forgive is again between him and Lucifer.