Sean is really the Candela character of all time because of how fundamentally entangled he is within not only the world but the literal mechanics of the game. He is entrenched in the cycle of violence and he has no capacity to see any escape.
And how could he? Why would he care in the end about monsters living among them, wearing human faces? There's nothing anymore monstrous about the shapeshifters than the men in that room—himself included. The only difference Sean sees between his doppelganger and those men is that the doppelganger is going to give him what he wants.
In fact, this is also the only difference he sees between himself and those men. He doesn't kill them because they did something monstrous. He kills them because in his estimation, with all of them as evidence of this belief, the only things you get in this world are the things you take for yourself. Their deaths won't bring back his brothers, or erase the things he himself did, or even really further his efforts to rescue his mother. He kills them because he wants to—if the world is inherently violent, and it is on a fundamental level, then he's going to take what he wants.
Because the violence of the world is baked into the fabric of reality, both narratively, through the Flare, which can never be defeated, only struggled against, and mechanically, through the significant odds of failure or complication. There is so little success in the world of not only Newfaire but Candela Obscura itself.
It has nothing to do with who is the biggest baddest monster. Sean's approach to violence and later betrayal is beyond the consideration of morality, because the struggles of Candela Obscura leave so little room to split hairs over morals. Survival is at stake. The organization of Candela Obscura is misguided and ineffectual not because of any inherent problems of the organization or corruption of its members, but because they are, in Sean's mind, always only making losing bets.
In Newfaire, the dice are loaded, the house is all-powerful, and humanity is the underdog. Was it any surprise that Sean Finnerty got tired of losing?
