He discovered that between the poison mark and the branded poison, he could use them against each other to help refine his body. In doing this, not only would he get stronger, but both poisons would weaken one another to the point where he wouldn't need the extra help.Wasn't he unable to refine the demon poison spot by himself ? Back then, he needed either a Dou Zun or a new essence flame and I don't recall him asking help from any of the Dou Zun he encountered... Unless the earth core bead counts as a new essence flame ?