Just imagine Dean kissing Cas for the first time, and Cas being completely shocked -

And Dean taking advantage of the momentary lapse to stab Castiel with the angel blade; the demon laughing as the life fades from those blue eyes.

eta: since you asked for a fic

WTH WAS THAT OMG NO! I went from smiling to my heart being ripped out and stopped out on the floor. I IMAGINE ALL THE THINGS AND I JUST HAD A MENTAL IMAGE OF THAT WHYYYYYYYYYYYYYYYYYYYYYYYY *cries*