Chen Ke finally figured out the source of his ability.
After the Blasphemous Dagger pierced Lin Mo's heart, Chen Ke had traveled through time. The soul that was supposed to be taken by the contract demon was trapped within Chen Ke's soul due to the effects of the time travel.
The Blasphemous Dagger's inherent abilities of "crossing through time" and "connecting with killing intent" were also absorbed by Chen Ke. Such a coincidence was unlikely to occur a second time.
The ability to absorb killing intent replaced normal human lifespan. In other words, Chen Ke's lifespan was essentially Spiritual Energy; he was a Blasphemous Dagger that needed constant recharging.