Sometimes I forget just how dedicated to his job Butler is, but like...that's a hardcore part of his character.

Book 1 it's obvious, he breaks a man's hand without looking and without second thoughts. Sure, "scared is better than dead," and he shows a soldier's honor when dealing with LEP Retrieval One, but he still obliterates them with only a moment's hesitation.

Even Book 4 makes sense ("But Butler was only paid to protect one thing, and it was not The Fairy Thief.") He's lost his memories of the People, of course he'd default to "Protect the Principal."

But then we see his job take precedence over everything else again and again even with his involvement with the fairies. Book 3, that sound grenade could have easily killed or maimed people. He didn't hesitate to use it. Book 5, even with his memories of the fairies intact, owing them his life, and knowing the risks of letting the demons be discovered, he is unapologetic about his thoughts on the situation: "If I had to walk away from here, it would not trouble me unduly, so long as you were walking away with me." Would he have some regrets? Possibly. Would letting the demons be discovered introduce new difficulties to his job? Most likely. Would he let that stop him from walking out and getting Artemis to safety given the first opportunity? Not a chance. Book 7, his baby sister is in potentially mortal danger, and he still prioritizes Artemis's safety over his own convenience in rescuing his family.

Butler willing to help others, would go to the ends of the earth for those he considers friends, but at the end of the day, his job is to protect his Principal, and that is always always his highest priority.