I'm going to name a few now.
One of the more disturbing deaths is from the villain of Dr. Favilier. Using Satan and demons as his "friends" he finds out a terrifying and horrible death when Lucifer and his gang pull in him into Hell for an eternity of torment. Watching his fear, you know how real this moment is. (Princess And The Frog)
Judge Claude Frollo. He's absolutely devious in his "righteous" beliefs of what he think is right. It makes him one of the ultimate villains. He thinks he is doing right, but for the wrong reason. His death scene when he see's the true beast of the Devil is his realization moment that HE is in the wrong. The Lord won't accept him. (Hunchback of Notre Dame)
For the Crown! Clayton was a formidable foe to Tarzan and his family. He has the makings of a true crook in doing again, what he thinks is right, and sadly probably the main mindset of the time. His death is all the more disturbing. Tarzan shows him who the real man is... (Tarzan)
Scar is simply trying to take his place in what he feels is his to do. When Scar's plans fail, he is literally torn and eaten alive by his former henchmen. Talk about a slow and painful death. (The Lion King)