I didn't want this he acknowledges for the first time. But, with everything already set into motion he knows he must play his part. The part of the always dutiful son, even with his father absence. Yes, he must play the part, however, it doesn't mean he is going to believe it.
His father ordered him to cast out his brother. His most beautiful angel in the darkest, furthest place from Heaven. He knows he must let his brother burn alone in his cage. It doesn't mean he's not going to stop it. How to break your brother from his prison while still following in his father's wishes? Easy. His father already laid the plan. They shall meet for the final battle. That is what he will be working toward. And if the plan pans out faster than it was set who is around to stop him?
When he hears of his angels, yes his for he is the commander of Heaven now, fighting to stop the seals from being broken he merely misleads them. There are always casualities of war. Of course, he could've sent his brother's to hell to save the righteous man, but that would mean his brother would forever be instilled in the cage. He doesn't control destiny, after all, and destiny says he and his brother shall face eachother again. Who is he to argue?
And so he lets the demons break the seals. Watches as the younger Winchester becomes corrupt with the blood of demons. Watches over his vessel, gives him Castiel. He misdirects his angels. He allows the corruption. Nobody ever suspects the corruption to come from the highest power. When the so called traitor is killed he doesn't feel anything for being the one to put him in that position. Nobody ever questions his orders, there is no doubt, that he is just. He lets more seals be broken, he lets more of his brothers die, but none of the pain of losing them could even compare to losing his brightest light. He plays them all as easy as he sings praises of his father.
When his vessel says no he understands. Sometimes he wonders the outcome of him saying no to his father, but that was not his destiny. He diverts to plan B or in his case, plan A, plan Adam.
Although he isn't at full strength without his true vessel and he could lose he finds he doesn't care. Not because he is a cold, warrior of God, but because he can finally see his brother. He can smell the ashes that cling to his grace. He can imagine the way his light is, not dimmed, rather just a different shade. Most of all he can feel his brother's warmth for him under the cold hard shell of Hell. He knows the games that they play with each other like the back of his sword. They played them all so very well. And if the Winchesters want to stick him in a cage with his brother for eternity at least his Lucifer won't be alone anymore. As he falls into the locked box he thinks his true vessels words will always ring true for him as well. "I'm not leaving my brother alone out there."