Lucifer was Gabriel's weakness. He'd have given anything, done anything to save him, but in the end, nothing he did had helped. Lucifer fought Michael, Lucifer fell, and Lucifer was trapped in the cage. And Gabriel was powerless to save him. He had suffered every single day after that, the years making him bittersweet and lonely. The passing days served only to help him distract himself. Until he met the Winchesters, he had almost succeeded.

Their arrival brought back the memories Gabriel had tried so, so hard to bury. And when he learnt that Sam was trying to save Dean from hell, he couldn't help but draw parallels, one in particular. Both were trying desperately to save their older brother who had doomed himself to Hell for eternity, a fate that was almost certainly inescapable. And the pain returned, harsher than ever, tearing Gabriel apart from the inside.

That was why Gabriel killed Dean 100 times over. That was why he so desperately tried to convince Sam not to keep trying to save his brother. Because Gabriel had been where Sam was, and Gabriel had lost the one person that meant everything to him. Because that loss had turned Gabriel into a killing machine, and he didn't want that to happen to anyone else, because he knew, Gabriel knew, exactly what that did to a person, whether they be angel or human.