This this and I wish more people noticed as much ngl. Ryuk has no morals by either realm's standards. Ryuk would kill his own kind if he could and he fully admits as much out loud and everyone misses it! Ryuk is an authentic nightmare hiding behind a startling ability to play cute and mostly-harmless (and also being ass-over-teakettle in love with Light Yagami, but hey, nobody ever said being evil made you proof against being besotted, though in Ryuk's case it did at least make him proof against dying over it...)
It endlessly fascinates me too, on this topic, that Ryuk successfully coasts in Light's moral blind spot for the entire series. Like, Light is closer to him and talks to him more and spends more time with him than with anyone else, but we never see him morally challenge or interrogate Ryuk about anything Ryuk says, implies, or might in the past have done, despite the fact that Ryuk is so obviously treating everyone's suffering as a sideshow the whole time. Even when Light finds out Ryuk stole Shidoh's notebook - with the implied potential death sentence to Shidoh! - he's only pissed because of the impact on his plans and he forgives Ryuk in a heartbeat, never commenting on the moral aspect at all. Even though "Kira", if he was even paying lip service to his own principles, should not be okay with Ryuk being Ryuk.
And yet Light is, always. I do wonder what loophole he carved out in his mind for that. Does he overlook Ryuk's sins because Ryuk never actually kills anyone innocent in front of him and Light can just tell himself thought crimes don't count? Is it because Ryuk is always so openly admiring of him and goes along with all his schemes, and therefore is hiding under the same umbrella of "it's fine as long as it's for the greater good" that Light uses to protect himself from his own hypocrisy? Is he using some kind of "it's his nature, he's a shinigami, he can't help it" internal excuse?
Or is it just that Light can't bear the idea of passing judgement on his emotional support monster, and so he just Will Not Ask about Ryuk's moral standing or past sins? I kind of think it's that last one tbh. (I do love the idea of Rem, in the brief window where she has chance, trying to warn Light about Ryuk. Like, "you shouldn't trust him so much, do you know what he's like?" ...but, of course, Light would never listen.)
But yes one of my favourite things about Ryuk is that he's so good at being absolutely evil but not in any of the stereotypical ways you'd expect for someone with his appearance and backstory. His claws are always clean. He doesn't really push boundaries or threaten or ever take anything by force, he never stands on his pride or dignity (except, like, that one time with Matsuda which I think in itself says something lmao). He does a perfect job of playing the mostly submissive, compliant, friendly pet monster, and nothing can ever quite be blamed on him because he never told anyone to do anything and indeed is always the first to ask "are you sure about this?"
And yet he still leaves ruin in his wake everywhere he goes but can always blame it on someone else. Like I've said before, it takes skill to play "the devil made me do it" as your excuse when you are in fact the fucking devil. đ¤