Note: I know it doesn't matter in the slightest but, I wanted to say that I actually wrote this drabble before the last episode of season nine was aired.
Disclaimer: I own nothing.
Killing things that need killing. Isn't that what we do?
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This is one of the tiny things that prove that Dean is changing. Slowly but inexorably.
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It used to be save but now it has become kill.
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Sam can see it, because even if he pretends not to care that much about Dean, he still tries to watch over him. The Mark of Cain is changing Dean so deeply in his core that Dean himself doesn't notice anything. Or maybe he does.
But, anyway, that's partially Sam's fault too; he can see that what he said to his big brother weeks ago still weights heavily on his mind, that it has weakened his psyche, made him vulnerable to the Mark's influence.
o0o
Now the Blade doesn't even need to be in Dean's hands to make him feel the bloodlust. All he needs is a weapon and a target to kill.
Dean feels a bit... distant towards that bloodlust but on the other hand, he's terrified because he knows that one day he will be unable to resist its attraction and the first person he'll kill will be the one nearest to him at the moment and it will most likely be Sam, and although he doesn't care about his big brother anymore, it is and will always be Dean's duty to look after Sam.