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.

They actually deal with the shitty things the animorphs did. They killed unarmed and helpless enemy combatants out of spite. They recruited mentally and physically disabled kids to their cause, only to later use them as decoy cannon fodder. There’s a time skip in the final book. None of them remained friends, the leader has a form of PTSD and is disgraced, his crimes having been exposed in a trial in The Hague (not his trial, the main bad guy’s, but the leader’s crimes were used as an argument by the defence). The last page ends with 4 of the 5 remaining animorphs implied dying by ramming their a spaceship into a superior enemy.