I love the style and cleverness, but I do not like the plot:
Cuphead goes to the Devil’s Casino and loses his soul. To recover his “soul contract,” he goes to work for the devil collecting soul contracts from the devil’s runaway debtors.
Therefore, YOU, as the player, are working for the devil and it is all fun and games.
Spoiler: It treats the subject in a comical way that might be confusing to kids (not that it’s a game for them), but... At the end, you have the choice to follow through and give the devil the contracts which ends the game and gives you the bad ending where you spend eternity in Hell or you can refuse to give the devil the souls which leads to one last big battle against the devil himself. You eventually vanquish the devil, freeing the people whose soul contracts you collected, promise to be good from then on, and get the good ending.