Okay first of all, Sierra didn't want to kill Pterano at first. He wanted to get the children for he blames the children that the "Stone of Cold Fire" didn't work. But obviously, Pterano wouldn't let that happen. And he did attempt it once or twice (example: "I'm sick of those whiny brats jumping all over me like a frog"), but he knew the children would tell their parents and the whole Great Valley would kick him out which was exactly what the children did and what the grownups did. So he had to think of a plan that wont get him kicked out of the Great Valley, which took at least two years to come up with. He wanted to kidnap the kids and frame someone for it. But for that he needed Pterano's help, for Pterano could actually earn the children's trust without threatening any one. Sierra wanted to earn everyones trust and then wait til the right time to strike. Pterano refused to help, because he knew that would hurt the children or worse. Being the good guy at heart he really is, he challenged Sierra into a fight to prevent that from happening. Does this all make any sense?
BTW, I'm glad you asked this. I just came up with that! thank you!
