Ever think about how technically the younger brothers are more scared of Mammon than Lucifer?
I swear this is canon, listen
Lucifer's sadistic. He's easily angered, and he always retaliates. And he makes sure his retaliation always takes the form of cruel and unusual punishments. His brothers are scared of the consequences but they're never scared enough to actually stop
Mammon, on the other hand, is a pushover. He doesn't hold grudges, and his bark is so much worse than his bite. He instantly folds if someone aplogises after wronging him, and usually, he'll even apologise back despite not really doing anything wrong. Mammon's usually also part of whatever shenanigans the others are doing
And in spite of all that, the younger brothers all keep pushing and testing Lucifer. They're constantly doing things that they know will piss Lucifer off, and no matter what he does and no matter how much the younger brothers don't want to face the consequences that's never going to stop any of them from continuing whatever they are doing
But with Mammon, in the handful of times we've seen his younger brothers genuinely anger him, they stop or retreat instantly. Hell, there's even a moment in Nightbringer where Belphie is willing to keep pushing against Lucifer but instantly backs down when Mammon gets in-between them and tells him to stop
And y'know what? It's the Strict Teacher vs Fun Teacher thing. The Strict Teacher is always going to be like that, you know exactly what to expect when you piss off the Strict Teacher. Meanwhile, when you finally manage to piss off the Fun Teacher you know you've Fucked Up™
