Alec didn't know how long he stood in the abandoned subway tunnel. It could have been an hour; it could have been ten minutes. But it didn't matter. Nothing seemed to matter anymore to Alec. Everything had ceased to matter after the man he loved (the man he had grown to love more than the entire world) walked away as he stood there, knowing it was his fault.
Logically, Alec knew the blame wasn't completely his. Camille and Magnus had their share of it as well. But, God, it felt like it as he thought back to the pain in Magnus's eyes, the hurt expression on his face when Magnus said he couldn't trust him.
Alec knew what he did was wrong, (Raziel, did he know) but he couldn't help it. At first, he had hated the thought of how Magnus's immortality would separate them, how Alec would grow old and ugly and die while Magnus remained young-looking and beautiful forever. He had hated how much he knew his death would hurt Magnus. He was jealous of the lovers Magnus had before him and would have after his demise.
Camille had known this and had zeroed in on his weakness. She tempted him with a seemingly perfect solution: to take away Magnus's immortality and let them grow old together. Alec had soon realized how wrong the idea was, but he had continued to meet up with Camille because, well, he was curious about Magnus. He wanted to know more about him. After all, one didn't often date someone who had, quote, "been alive when the Dead Sea was just a lake that was feeling a little poorly."
But Magnus never talked about his past. Avoided it like the plague, really. It frustrated Alec to no end. Magnus knew everything about him, from his favorite song to how he sometimes hated his parents, and Alec knew next to nothing about him. He didn't even know how old he was, for the Angel's sake.
So Alec met up with Camille in secret, and in turn, she told him stories about Magnus's past. He knew it was wrong to meet up with his boyfriend's ex-girlfriend, but he couldn't help it. He hated feeling like he didn't know his lover. He hated feeling so trivial.
And then Camille had turned on him and told Magnus of his weakness, how he had at first wanted to take away his immortality. And now Alec was paying the price for his mistakes. He lost the man he loved more than anything, the man he would tear down the sky for, all because he was jealous and paranoid. All because he had listened to a spiteful woman's tempting words and empty promises.
Alec stood alone in the abandoned subway tunnel staring after a man long gone, unaware of the tears dripping down his face (but very aware of the pain in his heart), and realized far too late how foolish his actions were… and how harsh the consequences.