There are a few gruesome deaths that I can think of in American animation.
Mufasa: I swear that when I first saw this movie in threatres, it was pretty intense to have him fall off a cliff and land in a stampead like that. It was sad, yet scary at the same time.
Scar: This was even more gruesome than Mufasa's death. He was attacked and possibly eaten by his former minions. And if they didn't kill him the fire probably did. While Zira did commit suicide in the sequel, I feel that it wasn't as intense or at least not as gruesome as either Scar's or Mufasa's deaths.
The Huntsclan: The scene where almost everyone in the Huntsclan are killed by being sent into a vortex like that. Not only was that so intense for a Disney cartoon, but the scene with Jake and Rose just makes me cry so much.
The Joker: Now the Joker's death in Return of the Joker is by far the most intense death scene I've seen. I believe that I saw the uncut version a few months ago and his death was just so scary and intense. The way Robin knew that he killed the Joker, even though he was just trying not to kill Batman, put so much more moral pressure and scaring on him. I think that I remember the cut version when it aired on CN once, but I think that the uncut version has more power in it than the off-screen electrocution. Besides that, it was just really scary to see the Joker actually die by his own gun, which looked like a toy gun too, then cheat death by taking over Robin's body and then die again. Now that was intense and gruesome.