Metatron had been patiently waiting. Hiding and waiting for the opportunity that had now presented itself. He was not stupid, he knew that the Archangels would have put aside their differences to smite him into atomic particles if he had attempted this while they still held power.
This was the one thing that could have gotten them all on the same page. Lucifer would have given up his hatred of humanity, Michael would have put his duty to father on the back burn, Raphael would have gone completely on the warpath and Gabriel would have barreled through the pearly gates with enough force to flatten them. And Metatron had witnessed first hand what happened when four Archangels focused their power on a single goal. Just ask anyone who had been around before the Great Flood. Oh that's right you couldn't, because those folks had been the recipients of all that power.
But now Metatron had his chance. Michael and Lucifer were both locked up tight in the cage and Raphael and Gabriel were as dead as the proverbial doornail. There was no one in heaven that could stand up to him. Naomi had tried but in the end, even with the authority of Michael's office behind her she had fallen and fallen hard.
And Castiel? Poor gullible sap was putty in Metatron's hands. It was easy enough to convince Castiel that he was undertaking some great cause and closing the gates of Heaven. Only Metatron could read those tablets. Well Metatron and the prophet Kevin Tran. It was why Metatron had to move so quickly; the prophet would soon find out the truth, that what Metatron had just done was a spell.
Thank Father for the Winchesters who provided the perfect distraction with their determination to close the gates of hell. Of course Metatron knew that Sam Winchester would die when he succeeded. The Angel tablet and demon tablet outlined trials that were meant to be undertaken simultaneously. The two who undertook the trials together would shield each other from the effects of undertaking the trials. Metatron had seen how close the brothers were and he was sure that Dean Winchester would be devastated by the death of his brother.
Yes, Metatron considered all of these things as he watched the angels fall. What he didn't consider was the effect of the grace of thousands of angels being stripped from them at the same time. No more than he considered the power of human belief so as he smugly watched the the last angels fall to Earth he turned his attention to other things, like preparing his library where he would store all of the stories of mankind.