I have an updated guess as to what Metatron is up to:
He resents being an angel. Whenever he talks about it he seems upset about being immortal, about not having died as a human. I really believe he wishes this endless monotony and duty to end. What if all he desires is death? What if the only way for him to die is to be killed by a more powerful angel? Given that he obtained immesurable powers by now there are few stronger than him. Raphael might be, but he does not have the air of one killing willingly. Michael would not do him any favour. Some chapters ago Metatron mentioned about Uriel “only Michael at his best form was able to stand up to him”. His current action turned a lot of people against him: Michael would surely love to see him burned to ashes, but to be strong enough to accomplish that he has to rest. Uriel got pulled into the whole mess, growing more and more irritated. Controlling Sitri might turn him - whenever he gains his consciousness back - against Metatron as well.
What I mean is: Metatron is deliberately making powerful enemies.
Of course, this could all be bulls**t, but it is a possibility.