I'll pray you'll call my name instead of turn away

Diavolos internal conflict of interests both as the future king and as an ordinary demon is so strong that yesterday I couldnt stop thinking about the fact that Lucifer is on the same place as his kingdom in his list of priorities, if not even higher

This bro literally let war criminals into his kingdom because his supposed-friend fell and asked him for help and that alone (and his loyalty) was enough for Diavolo to not only resurrect his sister and make her human (how the hell strong are you actually?) but also give them a home, and then do everything possible to make this home comfortable and safe for them as its shown in NB, even if it goes against the interests of the Devildom, his Kingdom

Forever mine favorite dialogue between him and Barbatos in Hard Lesson 18-14 in NB:

We all know that Diavolo takes his role as future demon king very seriously and he constantly doubts himself whether hes going to be a good king or not, its important for him to become a better king because it directly follows from his conflict with his father, he wants to prove to his father (and himself) that hes worthy to be a demon and a future demon king. But then the brothers show up - no, Lucifer showed up first - and the feelings he has for them all go against his established image of what a king HAVE to be. A king should worry about his people and kingdom first, but Diavolo worries about the brothers, and this makes him doubt himself again.

He also gave the brothers the right to choose where their home would be after Michael in Raphael's body said they could be accepted back into Heaven. Although, in theory, Diavolo should have stopped them immediately, because Lucifer already swore his allegiance to him, and he had already accepted them in his Kingdom.

Pretty naive for a future king, isnt it? Save everyone, even war criminals.

I like the fact that throughout both games we see him put the brothers before himself. And, of course, Lucifer is always first - because he is the firstborn, and he is the eldest, and because for Diavolo he will always be in first place. Because for Diavolo, Lucifer is a priority, something that even Simeon hints to him about in one of Hard Lessons of OG season 3.

Diavolo is a good prince and he will definitely be a good king, but having such a strong internal conflict really makes the relationship between these two complex and multi-faceted (because for some reason we mostly only pay attention to Lucifer in this regard, even though Diavolo is no less important in revealing their relationship, and only makes their relationship deeper and more interesting). This conflict also leads to him both being ready to lock up Lucifers brothers if they are a threat to his kingdom (Beel, Belphie) and being ready to fight the House of Lords for Lucifer, knowing full well that both his and the brothers' positions are precarious to say the least, and he will only make things worse.