Based on the Medicine King's intent to kill him, there was no way he could let the Medicine King live.
The matter the Medicine King mentioned was something he had already considered, but he learned from the Medicine King's memories that when the person who signed a soul contract with the Heavenly Flames dies, the flames not only don't disappear but become easier to subdue.
To survive, the Medicine King was speaking incoherently, spouting nonsense.
The greatest gain from the soul search was threefold: the location of the Heavenly Flames, powerful pill-refining techniques, and information about Yan Xiao's fire seed constitution.