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"Dean." Cas said in a voice with as much authority as he could muster. "Put the knife down and step away from the boy" he added in the same tone
Dean held the knife close to the throat of the unlucky victim, who had simply been in the wrong place at the wrong time. The boy had let out a small whimper, This had only made Dean more enthusiastic and he held the knife tighter until tiny drops of blood dropped down the boys chest
"Dean." Cas repeated "If you do anything other than let him go I will not hesitate to kill you" Cas said, backing up his threat by showing Dean the demon blade which he had behind his coat. This caught Deans attention he smirked sideways and his green emerald eyes quickly transformed into a dark black swirl
"That's a lot of big talk comin' from and angel with no Grace" he chuckled. "But you know you can't Cas you'd never kill me and we both know why. But, I'll put your little warning to test" he smiled while his eyes reverted back to green
Before Cas could even comprehend what Dean had said. Dean quickly slashed the knife in one long motion along the boys throat. He let out a painful strangled scream and fell to the ground. Cas looked at him wide-eyed while Dean didnt seem the slightest bit phased
With that Dean extended his arms outward leaving his chest and weaknesses' vulnerable. He stepped so close to Cas that he could feel his breath on his face. Dean took. Cas' hand with the blade and placed the tip right over deans no longer Beating heart "Do it Cas, I'm not stopping you anyway" dean said once again flashing his award winning smile. He leaned in even closer "But we both know you can't" he whispered. "You'd never kill your old pal Dean Winchester" he added smugly putting air quotation marks Around the word pal
Cas sighed "you're right, I could never kill Dean Winchester" Dean chuckled about how much easier this was than he had thought, Dean slowly loosened the angel blade from his belt And prepared to strike. "But" Cas added to his previous sentence "you're not Dean Winchester" he sighed "The real Dean Winchester died along with his humanity when you took over."
With that Cas plunged to blade into deans Chest "you're just demon with a familiar face" he added right before twisting the blade and finishing the job Whatever life was left In that body had disappeared before his eyes. Cas took his final look at Deans eyes before he pulled the blade out and let him fall
Cas stood there for a good minute until it had hit him that he was really gone. A solitary tear rolled down his cheek and he made his final announcement on angel radio before he could feel the last few ounces of his grace burning up.
"Dean Winchester has been lost."