Terra almost smiled once Xehanort accepted the extra coat, but his question just made him... pause again. And frown thoughtfully. How could either he or Isa answer that without opening a can of worms?

Carefully, he chooses his wording, and then replies. "From what another of their number told me a while ago, it isn't the people as much as it's the one in charge. He told them he was trying to help them, but he was only trying to trigger the destruction of the universe."

There. That... should hopefully drive the point home. It covered what he had heard from Axel in Promenade about Xemnas, and what he knew would happen if Kingdom Hearts were ever to be summoned, while explaining why being associated with such a group (even by proxy) would be a bad thing.