Idiots.
They let him live. They let his goddamned apprentice live. They put him in an interdimensional prison, are they stupid?! That won't last a month! And then it'll be wonderful, he'll be back and they'll all have to face up to the fact that their ideas of right and wrong failed them. Or they'll be dead. Villains always come back, it's our lot in life to escape every prison and death sentence possible and never reform. He'll be back.
Idiots. All of them.
Idiots. All of them.