Come on. It's obviously Trelane.
When we first met him in "The Squire of Gothos," he was just a petulant, spoiled child.
Now, he's an angst-filled, surly teenager, brooding about the way his parents punished him for mistreating humans.
"They sent me to bed without any supper. And now... I will have my VENGEANCE!!!"
Those were the same reasons why I chose Charlie X.