ignoring the "killing a few to save many" idea, why would I, the hero, YOU idiots summoned, want to help you after finding out about you leaking info to set my friend up for death.
for that matter, it seems like they just wanted to get rid of a potential loose end because if all they were going to do was spread false info they didn't need to involve the kid in the first place and could have picked any group of travelers to implicate.