If there was one thing that Tristan knew about his cousin It was that Zachariah was selfish. He was so self-centered that he was unable to feel worry for anyone but himself. There was no one out there in the world that they could threaten in a way that would make Zachariah feel actual pain and anguish.
Not his mother and most definitely not his wife. The only way to get Zachariah to feel true fear was to threaten Zachariah himself. According to Peter and Ariel, madam Mo was the exact same way which made a lot of sense.
Tristan had never known his cousin to get close to anyone, but it seemed that he and madam Mo got along like a house on fire. It was only fitting that their personalities were similar. So to get at them. Tristan had decided to use a monster of their own making. The hitman.
