Complete Monster: While Pete has always been depicted as something of a Jerk Ass, here, he basically used the King's illness to install himself as a tyrant, and when he discovered that Mickey and the Prince had switched places, he threatens to kill Pluto if Mickey doesn't agree to be his puppet king, and plans to murder the rightful Prince to ensure that he remains in power. Never has he ever been depicted as being so utterly selfish, ruthless, and downright evil as here.
Also to note, the King dies immediately after Pete finds out about the switch, and right after Mickey hears the King's last words, Pete confronts him, knowing that the King is dead, and says "Now that our dearly departed King is out of my way..." It may not have been intentional, but the timing and lines imply that he might have had something to do with that.
He Really Can Act: The film may only be 25 minutes, but Wayne Allwine utilizes them to really bring out the emotion in both Mickey and the Prince, especially in the scene where the king dies.
Moral Event Horizon: This version of Pete threatening to kill Pluto and abusing his power under the name of the king.
Tear Jerker: The old king's death. Not only does he die unaware of his captain's atrocities in his name, he just seems so broken, and he doesn't even know he's talking to an impostor and not his son, because Mickey doesn't have the heart to tell him.
It also applies to the Prince who didn't find out about his father's death until a passing boater informed him. The grief of losing his father was hard enough, but it was also coupled with the fact that he wasn't there to say good-bye. His long silence after hearing the news made it apparent.