User blog comment:NenMaster/The Fall of Phantom Troupe/@comment-48285-20130910052429

I believe Kurapika's vow was to only use it on the Troupe members. If he interrogates them one by one, then he will find out that not everyone was present during the Kurta clan massacre. That said, I don't think there will be anybody who would betray the other members of the Troupe. Just like Uvogin.