The reason why he did not use the "Spirit Seed Gathering Dragon Skill" on him from the beginning was because at that time he merely suspected Kim Dean and did not have concrete evidence.
Martin never went to Flying Stone Island, instead, he was hiding in a secret mansions in Megge Port. After Owen Bertram returned and dealt with Martin and this so-called Beggar Sect with a sweeping action, the evidence of various crimes found was even more shocking than what Felix knew!
This left Owen Bertram, on his way back, with a consistently cold countenance, reconsidering the question: Should we carefully plan out the strategies against East Prison Town Comfort Office?
For instance, this "Beggar Sect" committed countless heinous crimes. If these people were to be sent to the Yama Hall, they would probably be sentenced to the Eighteen Levels of Hell, and even decades of punishment would not cleanse their sins!