why did jack want to kill the alpha kids on prospit and derse?

If you’re talking about when he stabbed Jane here it’s because he was ordered by the Condesce to do that. If you’re talking about this page it’s because Jack was possessed by Caliborn and Caliborn told Jake he would help him ascend to god tier and become powerful. So he controlled Jack into blowing up the moon of Prospit and having Jake go god tier.