I don't know why Scar was jealous of peace and Mufasas's position as King. I wish he could be a better uncle to Simba and a nicer brother to Mufasa. The "Long live the king!" means "Goodbye to the King!" He even teamed up with his hyena henchmen to kill Simba and Nala to assure his rightful position as King. But he failed and gets murdered to death for his lies and betrayal by his own henchmen, and today, remains as a spirit until he was destroyed by the Great Kings of the Past.
It is unknown why the Kings destroyed him.
An est. answer is: He was destroyed because when he was still alive, the Kings found out he shouldn't be a spirit like Mufasa because of his behavior at the Pride Lands.