Castiel's eyes meet blackened green in a clash of wills.
Muscles strain as he struggles to keep the sharp serrated edge from his neck. A low hiss as the tip pricks hard, draws shining blood, beading and welling. A flash of memory from a darker place, a darker time: this is not the first time Dean has drawn the life from him. And again not for the first time, he curses the fact that Dean-- that mortals in general are so very susceptible to infernal workings.
He expected many things—but he never expected that the man who is fresh from the Pit and thus bears a hate of Hell and its deeds running far deeper than many of his brethren, angels who are actually sworn against the Pit, would have fallen so far, so fast as to allow himself to be possessed yet again by more of Hell’s work. But love is a strange thing that his Father has given the humans, and Dean is utterly reckless in that love where Sam’s welfare is concerned.
Castiel had hoped that his charge would still be able to recognise him for what he was (hope, good, salvation). As Dean's demon-forged blade rips across his throat and a knife enters his vitals, his last thought is that perhaps he should've wished for something else.