#kodlak

So, you have two guilds that both accept gold in exchange for hurting and/or killing people. What sets them apart from each other? Their acceptable targets, and their methods. You can't hire the Companions to kill your neighbor. To beat him up, sure. But not to kill him. We know this because you fail the "Hired Muscle" quest if you kill a target you were just meant to intimidate. When you steal items from shops or generally cause trouble, generic Hired Thugs show up to kill you, not Companions. The bottom line here is that the Companions are not assassins, or even mercenaries that will accept any paying job—just as they have standards for recruitment, they also have standards for which jobs they'll accept.

Methods: this is why Arnbjorn was exiled from the Companions, and why he joined the Dark Brotherhood instead. His methods were too violent for the Circle to accept. If the Companions considered extreme violence acceptable, he'd still be around.

His dialogue:

You are moon-born. You are wolf. So you're a Companion, then? I can't imagine you got your gift anywhere else. I hope you have better luck with them than I did. Oh yes, I was once brother to the Companions. Let's just say they found my methods "unsettling." The Dark Brotherhood, obviously, feels differently."

It's heavily implied here that Arnbjorn's tendency towards excessive violence made him a poor fit for the Circle. He had to seek out a guild of assassins in order to find appreciation for his brutal talents. The lesson we can take from this is that skill at killing alone is not enough to merit the Companions keeping you around: you also have to demonstrate a respect for the company's honorable ideals.

We also have evidence that the Circle keeps an eye out for these violent tendencies in their prospective recruits. The mercenary Uthgerd is barred from joining because she accidentally kills someone during training. Even in this violent world, the Circle isn't willing to tolerate such senseless killing.

Her dialogue:

You must really love to fight. "The heat of battle is the fire that forges the strongest blades. It's an old Nord proverb. That, and a true Nord never misses a chance to test her worth."

Sounds like you've got a grudge. "You been talking to those Companions? "Too hot-headed," they cried. Weak, pathetic cowards, the lot of them!"

Why'd the Companions reject you? "It wasn't my fault! I told them over and over that it was an accident! They wanted me to prove my worth, so they threw me up against a young whelp of a lad, hardly old enough to grow his first chin-hairs. I guess they thought a woman wasn't strong enough to hurt him. I didn't mean for him to die! Why would I want that? I just... lost control."

Judging from this interaction, the Companions are actually less hotheaded than your average Nords. They don't want mad berserkers in their company--they want disciplined warriors that don't let their emotions control their actions. This makes a whole lot of sense when you consider that anyone who joins might eventually become a werewolf. The Circle is aware that maintaining their code of honor is very difficult in the beast form, so they only accept warriors that already have a significant amount of self-control.

When you encounter the Companions, they're in the middle of a schism over this very issue. Aela and Skjor represent the side that says might makes right. They believe their Daedra-granted powers grant them leave to pursue bloody slaughter and revenge. This is a corruption of the company. It's not the way all Nords behave. In fact, the Harbinger is specifically trying to cultivate an atmosphere of cool resolve in the current batch of whelps:

Under Kodlak's watch, losing control of yourself as a Companion is seen as a sign of failure. It got Arnbjorn kicked out, and kept Uthgerd from joining. Perhaps most importantly, it's a sign that you will never be worthy of the beast blood.

Kodlak: "How are the young ones coming along?"

Farkas: "Some are too happy to fight. Blood runs hot."

Kodlak: "I remember when you were the same way. The more they train, the more they'll cool down."

Farkas: "I hope so."

Kodlak: "Just have them focus on the calm in the battle. Control the rage, don't let it control you."

Kodlak and the twins represent the classic ideals of the Companions: honor, integrity, and loyalty. So far they have maintained a tense balance between this code of honor and the violent temptations of the blood.

Dialogue on Kodlak's view of revenge:

Again and again during the questline, members of the Circle are punished for blindly pursuing violent solutions. However, it's also important to remember that their two great acts of violence towards the Silver Hand are both reactions to attacks on members of the Circle, and both attacks are on fortified encampments: Aela and Vilkas are basically attacking bandit forts, not villages of well-meaning werewolf hunters. The first time we encounter the Silver Hand is when they ambush Farkas at Dustman's Cairn. He defends himself and kills them.

Farkas: "Kodlak did not care for vengeance."

Vilkas: "No, Farkas, he didn't. And that's not what this is about. We should be honoring Kodlak, no matter our own thoughts on the blood."

Skjor and Aela alone are the ones who point you towards the Silver Hand fort and commence the slaughter. Given that Kodlak and the twins aren't involved at all, it's probably safe to assume they wouldn't approve of these bloodthirsty activities.

In fact, Kodlak is intensely worried that Skjor's reckless nature will get him killed:

Skjor dies because he rushes ahead and tries to wipe out a fort of Silver Hand on his own. When you tell Kodlak that you and Aela have been attacking the Silver Hand out of vengeance, this is his response:

Kodlak: "You're a strong warrior, Skjor. Your heart is fiery and that serves you well in battle."

Skjor: "Thank you, Harbinger."

Kodlak: "But even the bravest of men consider their actions. I want you to survive long enough to take the mantle."

Skjor: "Don't worry about me."

Aela and I work to avenge Skjor's death."Your hearts are full of grief, and my own weeps at the loss of Skjor. But his death was avenged long ago. You have taken more lives than honor demanded. The cycle of retaliation may continue for some time."

Kodlak dies because you and Aela provoke the Silver Hand into a bold strike on Jorrvaskr itself. You avenge Skjor by killing Krev the Skinner, but then you and Aela go past honor into something far uglier. Vilkas refuses to follow you in order to purify Kodlak's soul, because he feels his honor has been tainted by his own need for vengeance.

Vilkas' words:

"Kodlak was right. I let vengeance rule my heart. I regret nothing of what we did at Driftshade. But I can't go any further with my mind fogged and my heart grieved."

For the Companions, vengeance leads to senseless death and grieving hearts.

The DB questline is about the fantasy of being a morally destitute assassin. The Companions questline is about restoring honor to the Circle, and it treats revenge and violence very differently. These two factions are not the same merely because they're in the same game. The difference between them defines their driving themes and purpose in the larger TES world.

Compare this to the revenge you enact in the Dark Brotherhood questline. You can just slaughter Commander Maro in broad daylight on the Solitude docks, as vengeance for wiping out the Falkreath Sanctuary.

His extreme fear when you show up out of nowhere to kill him is treated as a joke—and this is GOOD, because that's what the Brotherhood is thematically about! Performing ridiculous assassinations on the innocent and guilty alike, and going home at the end of the day to your torture chamber. No punishment, no repercussions, no guilt felt by anyone in the Brotherhood. At no point are you asked to question the morality of your actions, because the story assumes that if you choose not to kill Astrid in the abandoned shack, then your character is fully on board with cold-blooded murder and assassination.

The behavior of characters in a certain setting cannot be reduced to the lowest common denominator of the most ruthless villains. These characters do have moral codes, and that can't be waved away by pointing to the darkest acts of the setting. Sinding is a werewolf that commits horrible crimes under the influence of his curse, and that makes him a very interesting fallen character--however, the Circle would never dare go near him, because he represents the ultimate failure that likely even Aela and Skjor fear: a total loss of control leading to the killing of innocents.

This is WHY the Companions are so potentially interesting to write about. They're in a constant state of conflict between their honorable ideals that place value on restraint and discipline, and the reality of their Daedric corruption that tempts them towards slaughter. Even when they surrender to this pull, however, we have to remember that it is only werewolf hunters that they kill. None of the Circle ever fall even close to the level of Sinding. If they did, they would no longer be fit to be Companions, and they would likely follow a similar path to Arnbjorn.