Marvel comics has always catered to an older crowd than most of their competitors, and they have never shied away from including adult themes in their storylines or killing off a main character.
Their heroes will kill, and death is no stranger to the Marvel Universe (she's part of a love triangle with Deadpool and Thanos!), so they have presented readers with some pretty grisly panels over the last 55 years.
And speaking of Deadpool-he's proven to be one of the grisliest characters in comics because he can regenerate from pretty much any wound, so comic creators like to really stick it to him.
Of course there's no gore quite like zombie gore, and the Marvel Zombies series featured zombified superheroes doing all sorts of nasty stuff, such as Spider-Man munching on Mary Jane or Giant-Man using Black Panther as a living food source. *shivers*