The problem they had was that the information David knew was too dangerous. As far as they (the animorphs) knew, they were the only people on the planet who were actively resisting the alien invasion, so if David wanted to, he could go to the aliens and give up humanity's only chance at salvation. I think in the end they just couldn't bring themselves to kill him in cold blood, so they went with that option. Although it actually came back to haunt them later.