Where did it say Snape created the spell? According to that Pensieve memory, James used it on Snape in his fifth year, but the spell was listed in Snape’s sixth year potions book— and Remus mentioned that it was “in vogue” for a while in his Hogwarts. The HBP potions book was what Snape took credit for creating, but not everything within it. The book had instructions for potions copied over with his notes, along with spells he created— and, in the case of Levicorpus (I presume), spells he heard in the halls that were “in vogue” at the time (Lupin’s words). Likely it was because the spell was widely used and he noted it down.
Moreover: James didn’t just use Levicorpus on Snape. He also threatened to take off his underwear in front of everyone, exposing his genitals. That is very much a step further than hanging someone by the ankle, which Lupin suggests was the norm (though that doesn’t make it any less horrifying).
And what does Snape trying to get Lupin expelled have to do with anything? I’ve said already that I am in no way saying Snape is a good person. His intense bullying from the Marauders does not excuse his mistreatment of children as their Professor. That doesn’t change the fact that what the Marauders did was also seriously fucked up.
Moreover: Snape trying to get Lupin expelled was stupid and Snape himself was undeniably a nosy, annoying little shit who was very hatable. Regardless, he didn’t know he might have died that night. Sirius did. He knew that Snape would succumb to the taunt, go down there, and probably die. He probably didn’t consider that Remus would have been expelled for it (at least). Even in the best case scenario, Snape might escape death, but still run and tell everyone (including influential pure blood Slytherins with powerful parents) that there’s a werewolf in the school, exposing and humiliating his best friend and possibly costing him everything, all for a good laugh (and that’s still assuming he meant for Snape to survive).
And this wasn’t the only time. According to him and Lupin, there were many “near misses” and they just laughed them off. Hell, even decades after, Sirius said “it served him right.” Lupin didn’t even disagree.
In my opinion: in this situation, Sirius was very much the one with the culpability. And none of the Marauders were decent people.
And again, that doesn’t mean Snape was good either.