Gordon, Gunn, Lenore, Dean: a hunter's choices.*
Gordon went to find and save his sister from the monster that took her from their house, under Gordon's own eyes. Gordon tried to fight for her, to defend himself and his sister, but against a vampire there is little one can do, unless you know how.
So, Gordon put his head down, stubbornly. Suddenly, he knew there were monsters out there who kidnapped innocents from their houses. He learnt. He became a hunter. He tracked down that vampire...found his sister. Only she wasn't his sister anymore, at least not in his eyes. She had been turned into a vampire herself.
Can you imagine the horror?
Gunn in Angel was exactly at the same point when he met Angel and the others. He was killing vampires right and left, as much as he could, because his sister had been turned, because he had to kill her. Sure, Gunn easily moved to stand beside the 'good guys', Angel Investigations, and in a later episode actually turned against his own ex-friends, who were happily keeping on killing each and any demons/monsters they happen to find, ignoring the fact that in the Angel 'verse, not every demon was actually evil.
Apparently, the same distinction has been made in SPN. In Bloodlust we learn that Lenore and her group have turned to feed from cows, in order to survive. And this is an important distinction to make: BUT it's not a question of right and wrong, for Lenore. It's a matter of survival. She's a vampire, she has learnt to control herself because otherwise she'd be putting her life at risk. Because otherwise a hunter might track her down, and kill her.
Lenore hasn't changed.
Lenore isn't, as some of the demons we meet in Angel, good to the core only with a demon look.
She makes a choice not to kill because at that moment in time it's the best choice she can do to survive. She states that in clear words. The implication is that in another situation, where their survival weren't at risk, they might make a different choice.
Gordon isn't that wrong in affirming that this choice of hers doesn't change who Lenore fundamentally is: a blood sucking vampire.
Gordon isn't wrong in affirming that Sam might become a killer. Andy's brother did. At this point in time we don't know yet if Ava might be, too.
At this point in time, we don't know yet if Sam might become a killer, too.
It's a possibility. John Winchester himself considered it as such.
But Gordon doesn't allow for shades of grey. Because if he did, he would become his sister's killer. And he can't accept that.
I understand. He's your brother {she was my sister}. You love the guy {I loved her}. This is gonna cut you hard {it did to me}.
Being a hunter is more than a mission, for Gordon.
Being a hunter is his way of survival in a world that has stripped him of his humanity. A world that has put in front of him someone he loved, his sister, turned into a monster. And he made a choice. A choice that Dean might have to make in the future.
Being a hunter is all that Gordon can be.
If Dean had to kill Sammy...the mere idea of it is unbearable.
Gordon is what Dean could become.
Gordon is what John hasn't become because he had Dean and Sam.
Gordon is a terrified man in a world of supernatural evil, a man who can justify his own existence, who can believe he is still human, only by maintaining a strict world view with no shades of grey.
But what if Gordon is right?
We know he isn't, right? But what if he is. What if he is?
That's why he's terrifying. Because he could be right. And Dean knows it. And John knew it.
Your dad, if it really came down right to it, he would have the stones to do the right thing.
Are you're telling me, you're not the man he is?
Gordon is what Dean could become. So, yes, he's terrifying, and he's brutal, and he kills - murders - with too much ease. But we've seen how Dean was after John's death, we've seen what he is/was becoming. If Dean had to kill Sam...Gordon could be Dean's future.
* As a vampire, Lenore is a hunter too.