What kills me here is that Cas so obviously ISN’T pleading for his life. The angel who risked EVERYTHING for one human, who turned his back on his own brothers and sisters, knowing perfectly well he would be hunted for it, and possibly killed, pleading to the one person he loves the most for his life? No, I can’t even begin to describe how out of character that would be.
No, he’s pleading for Dean. For Dean not to go on down this path, not to become the monster he would hate to be. It’s not too late, Cas is saying, please realise that even if you kill me, it’s not too late and it’s not your fault.
The even if you kill me, I’ll still love you is so obvious here, it’s in the way Cas doesn’t fight back, not even to just stop Dean long enough to get somewhere safe, it’s in the way he’s willing to die by the hand of the person he loves, and yet the only thing he cares about is pleading for Dean. Not for himself, but for Dean to know he can still fight it.
It’s not please don’t kill me.
It’s please don’t let the mark turn you into someone you would not recognize. It’s not too late. I love you.
Focus on the dream, focus on the details, picture them, feel them. The TARDIS will track on your subconscious and extract the relevant information. It should be able to home in on the moment in your timeline when you first had that dream.