Either real or delusional, Lucifer still was the lightbringer. Back in the days when his Father loved him the most, he was the brightest of them all. Now, times have changed. It seemed that, putting the sinful, unworthy monkeys aside, Father chose himself another most favoured creature. The child that yet stumbled upon his feet while he – the first and most devoted one – had already been loyal and of use for eons. And still, God chose the one who loved the vermin more than his Maker. The one that in the end, only needed a one tight grip on the imperfect, broken soul and its flesh, only needed very little words spoken by a human mouth, accompanied by a pair of eyes which for some reason, happened to offer a better scenario than anybody in the universe ever had, God Himself included. It was all it took to rebel against everything that was ever created or even simply planned to come.
Of course, there were other strong motivations behind Castiel's actions. But everybody knew that weren't it for the main one, his doubts would have been left unattended for another eras to come. And even if they were ever outspoken, their outcomes would have been but a breath of a dying butterfly compared to this. And so was Lucifer's fall and its consequences in comparison. His choices, when put next to the little Angel's, made him merely an excuse for a rebel. Castiel once thought of himself as a new, improved God. But if anyone ever cared to ask Lucifer, he'd say that what Cas did, made him a new, improved Satan.

Did anyone ever call Cas Satan?
Did anyone ever stop calling Lucifer Satan?
No and no.

The little piece of the first fallen, the very essence of his wrath which got imprinted into Sam's damaged soul and now has passed onto the greatest traitor that had ever walked the Earth and slaughtered Heaven finally found an accurate form of revenge. The soul of Sam Winchester was all along only a miserable consolation prize, a horribly inefficient way to release the anger.
This time it would be different.
Needless to say, Lucifer knew perfectly where to draw blood from. Driven and blinded by the same self-pity that made him fall in the first place, he could not see that yet again, his motivations were vile, his worldview poisoned and his actions selfish and uncalled for. But even if he knew, it would not make things any different. He didn't care, not anymore. What he cared for, the only thing he had, actually, was vengeance. And he would get his revenge, his wry equivalent of justice and he would savor it until the very last drop. In his mind, it was the most perfect possible creation, a machinery that was meant to backfire at the originators of his pain just as nothing ever did before.

Castiel could only think of himself as cursed. Until now, he never really was. Getting to him has always been almost amusingly easy. It was never really about the brothers that he murdered, not about playing God and spitting in the true one's face, either. For those were plainly the means to justify the cause.
The one thing that ever mattered: Dean Winchester.
And the one thing that ever mattered to his Father: love.
Lucifer would happily oblige to answer that calls.
Cas will get enough of Dean to choke himself on him. And feel enough love to make it into a length of rope to hang himself with it it.
And it all starts now.
*****

'Hello, brother!' he began, his voice cheerful in the way Castiel hadn't heard in ages.
After all, it was a long awaited reunion. His laughter was genuine, but they both perfectly knew what note lied beneath it.
'So you came back to fix it? Well I came back to fix you! Rejoice!'
Before Castiel could say anything in reply, assuming that he actually meant to do that at that moment, Lucifer snapped his fingers.
Just like that, the Angel collapsed heavily onto the ground.

Dean ran to him in an instant, not giving his actions a single thought, neglecting his freshly cured little brother in the process. He placed the vulnerable body of his friend on the now vacant bed and immediately checked for any vital signs. Cas was alive, technically speaking.
But whether he was still there with him, Dean could not tell. Losing him just like that again, after he only got him back? It was like life and God and his all damn Host pissing onto everything he loves. He won't let that happen. He cried out the Angel's, he shouted profanities just to come back to calling the name again, repeatedly.
Sam stood next to his brother. His face was marked with fatigue and heavy with sorrow. He did not however say a thing. Because he knew. Well, he could at least imagine what Cas was going through right now. The same was about Dean – Sammy knew, even without his brother saying a word about it, he would never admit it, anyway. So again, Sam didn't say anything. Not now. Dean was far from over, and if he needed to get it out of the system this way, so be it.
But Cas couldn't hear any of it. What he did hear, on the contrary, was the voice of his most despised brother, loud as a thunder and poisonous as a serpent, preaching: and thou you be the one that he sees you as, command that your flesh be made stone .
The Angel did not know what to make of this blasphemy yet. But he did have more than a general idea of its danger, judging from the echo of Lucifer's sly cackling.
Frankly speaking, he only thought he did.
He had no idea.
But of course, the lightbringer stood up to his name and brought the revelation eventually. One could consider it an act kindness, weren't it for the fact that the morning star decided to reveal his plan too late, on purpose. He would throw Cas some parts of it, just like you throw scraps meat at a dog at your mercy. And he would poison those scraps. Because it's funnier this way.