The devil himself has made multiple appearances within the Rick and Morty mythos with his first ever being in the Adult Swim series’ episode “Something Ricked This Way Comes,” and now the real reason Rick hates Satan has been revealed. In the episode, Rick torments the devil by turning all of the cursed items he is selling in a store in town into amazing artifacts of power without any side effects. By the end of the episode, Rick and Summer get super buff and beat the devil to a pulp. Outside of the realm of the television series and in the world of comic books, it is revealed exactly why Rick hated the devil so much.
In Rick and Morty: Go to Hell #5 by Ryan Ferrier and Constanza Oroza, Rick and Morty find themselves in hell, only Morty is on the verge of getting out. When Morty convinces Satan that he doesn’t belong there, the devil tells Morty he can either be released immediately or have one wish. Morty wished to have Rick be brought to the devil’s chambers as he was previously fighting for his life in the further reaches of hell. When Rick got there, he murdered the devil and gained all of the powers of the King of Hell. However, before Rick arrived, Satan tried to convince Morty to leave him behind and in doing so revealed exactly why Rick hates Satan so much.
The devil tells Morty that Rick is the worst creature to ever hold consciousness, and being the devil, he would know for certain that that is actually the case. Satan reveals that Rick is a truly bad person and he is perhaps the only being in the known universe who has that information. Rick is by nature a very mysterious person and doesn’t like it when anyone thinks they know who he really is, as indicated in the Rick and Morty season 5 episode “Mort Dinner Rick Andre” in which Rick gets angry with Mr. Nimbus after the King of the Ocean brings up something from his past. The only difference is, the devil doesn’t just think he knows Rick, he actually knows him as he has access to view into the souls of every living thing in existence, and Rick can’t stand it.
While the comic is the first time fans are aware that the devil knows that Rick is the worst living being in the universe, Satan indicated as such in his first appearance in universe. In “Something Ricked This Way Comes,” Mr. Needful aka the devil tells Summer that he is the devil, but that her grandfather is the devil, indicating with his tone that Rick is a terrible human being akin to the ultimate evil that is synonymous with the idea of Satan. And, if Satan calls you the devil, then you're probably really despicable.
While Rick proved his hatred for the devil in the Adult Swim cartoon series by beating him up with the help of Summer, he doesn’t express it fully until Rick and Morty: Go to Hell #5 in which he strangles Satan to death. Rick says he messes with Mr. Needful in the episode because he wants to prove how stupid the devil and his satanic powers are, fans now know there is a deeper reason behind that hatred that finally exploded within Rick when he killed him. revealed the real reason Rick hates the devil, and it’s because the devil knows how terrible a person Rick truly is.
