Kol Mikaelson could hear the voices of the Ancestors of the New Orleans witches telling him to kill Davina Claire. He fought against their voices and he fought against their compulsion, but he was losing ground. The blood lust rose in his throat and he pushed it back down each time out of love for his precious Davina, the one person who had made him feel alive again after a millennium of death. But he never was very good at controlling his blood lust in the first place. He had spent a thousand years giving in to every impulse that entered his mind and now exerting any degree of self-control was like trying to use a muscle long withered and weak. But Kol was motivated by love and a desperate desire to not destroy that which had brought him his greatest happiness.
But the voices and the pain pushed him to the brink, and then the blood pushed him over the edge. He sunk his teeth into his precious Davina's neck and drank deeply from her then let her drop to the floor of the Saint James. Kol dropped to his knees as blood dripped down his chin while he watched the red puddle spread across the floor from the wound he had ripped open in her neck. He heard her heart stop beating but was powerless to heal her. His Davina was gone, and the dastardly deed was done by his own hands. Kol wailed over her body as he held her then felt the Ancestors vacate his mind and return control of his body to him, leaving him to wallow in his misery.
Kol had just destroyed the one good thing in the world that made life worth living. It mattered not to him that the Ancestors pulled the strings, it only mattered to him that he was guilty, that he was weak, and he was at fault. Kol spiraled into a sea of depression and anguish and his only escape was to leave this world, too. When Marcel Gerard came into the Saint James to look for Davina, Kol took the opportunity to bait a now-grieving and enraged Marcel who bared his fangs and bit Kol, sending the deadly venom that would end Kol Mikaelson. As the poison spread through his body, Kol knew he would soon be truly dead like his brother, Finn. He welcomed that death as life was not worth living without his precious Davina Claire.