That's because-- [he pauses, uncertain. if he tells him he's responsible, Vash will understandably be afraid of him. but on the other hand, it's valid, that fear. he shouldn't be near Livio.

but he also shouldn't be alone. and so, voice breaking slightly, he tries to come up with an answer]

Because some bad people wanted to hurt someone, and they knew if they went through the kids, it'd lure him out.