I think it's super interesting that, even though we (or at least I) like to talk about the cannibalism in Mairuma as a representation of the tension and everything else Kirio and Iruma have going on, it technically isn't cannibalism. Or, well, it exists in a state of both being and not being cannibalism.
I should be clear here that cannibalism is a social construct and it has been practiced for a number of reasons throughout history, for better or for worse.
Moving on, what happens in the manga isn't cannibalism in that cannibalism is the act of one member of a species eating another. Additionally, it's not cannibalism according to demon society, which encourages the consumption of humans.
However, it is cannibalism in that the manga doesn't exist in a vacuum, so that there is still this transgression where someone that we the audience recognize as human--as being sapient--wants to consume someone else who is also recognizably sapient.
So, we have Schrodinger's cannibalism.
Except, even when it isn't cannibalism in-universe, it sort of is. Because, though it is culturally accepted for demons to eat humans, Iruma has already been recognized as a demon, so eating him is...messy.
In terms of the story, the cannibalism is super important in that it's a breaking point. Iruma has a wonderful life in the Netherworld, surrounded by people who love him. He's coming closer and closer to viewing himself as a demon. Then Kirio (Iruma's example of a 'true demon') shows up and declares that he wants to eat Iruma. Because, according to demonic society, Iruma is only as valuable as the power he provides to others and his life is made for consumption. So, he's facing this major cognitive dissonance between his parents (who were supposed to support him, but only used him) and his new home (which has supported him, but is supposed to use him). He's going to have to face the fact that the place he loves doesn't view him as a person.
That, of course, doesn't justify the cannibalism. The Victorians (and earlier Europeans) ate mummies because they wanted to be healthy, but it was also a sign of them viewing Egyptians as subhuman.
The rhetoric surrounding the cannibalism is also very fascist. It focuses on Iruma's 'place', and is, in some part, Kirio's attempt to reassert his own worldview via invalidating Iruma's. Here, I'm defining fascism as a restrictive hyper-nationalism, wherein only those who fit within pre-defined roles are permitted to succeed and also promote oppression of those who do not fit within those roles. In this view, Kirio is the victim of fascist system; he did not fit into the system until he forced himself to fit into the system. His despair worldview, his madness, is him coping with the ugliness of the world. If he were to accept Iruma's worldview as right, he would have to deal with the fact that he was wronged and that no one saved him. Eating Iruma would prove that Iruma's refusal to despair is just a 'human thing' and that it is invalid. Moreover, it would be Kirio once again justifying his own trauma by forcing Iruma to accept despair as good (because hurt people hurt people).
The 'other species' thing also doesn't work out completely in that humans and demons have at least been shown to have the capability to hybridize. I know that sci-fi and fantasy love to play loosey-goosey with lines between species, but the point still stands that these two species are close enough to crossbreed.
In summary, cannibalism in WtDS!IK is super loaded and fascinating and I love this story and cannibalism in general is super loaded. I didn't even talk about the erotic aspect of it.
(I talked about some of the cultural stuff surrounding cannibalism, but it's genuinely fascinating to look at how it is so dissonant in different cultures. Also, if you've read this far and have a strong stomach, look up prion diseases (especially kudu [this is the big one] and mad-cow disease).