Kol had never wanted to become a vampire, he had never wanted to give up his ability to do magic. He had been a good son and from what he remembered a much kinder person before he was turned, but then their Mother had forcibly turned them into vampires. She had taken the choice away from them and taken away his link t magic; she had stayed human so why couldn't he have? Why did he have to join his siblings in this eternity of being monsters?
And so he flipped something inside of him, something that caused a darkness to overcome him and made him feel like the things he did wouldn't matter. Kol Mikaelson was the first of his siblings to flip off his humanity, and non of them even realized it until 200 years later when a turned vampire did the same. He turned off his humanity, because if he couldn't use his magic what was the point? He had no escape from this either. He had no way to end the existence–so his subconscious mind protected him.
He had had plans, he wanted to become the best witch he could be–he wanted to be powerful enough that he could protect his family so that their Mother and Father would actually see him rather than lumping him in with his siblings. That dream was shattered though. And it wasn't until nearly 400 years later that his humanity ended up back on, and it was never known what had even caused it. But it was torture to him.
He didn't have time to really know what he was, he didn't have time to find his new self before that switch was flipped and so it became a defense mechanism to always had it at least half on. He didn't want to deal with all of the things that he had done, he didn't want to come to terms with them or feel like he needed to atone.
But when his mother brought him back as a witch, that wasn't there. His switch hadn't been fully off for nearly 300 year though so it was easier without it than it had been the first time, but it was still a few days before he began to listen to what his Mother said and begrudgingly help her with her plan. You can't ignore Mommy Dearest for to long after all.
He met a few other's after that, managing to pull himself a bit further up as he built p witch contacts; and got relatively back to his normal self. Then he met Davina Claire, and oh was there something about her. Something in her defiant look as he flirted with her, the cocky look in her eyes as he tried to make her fall for him. It drew him in and he was suddenly glade that he had managed to live for all of these years if only to get to this point.
Then Finn cursed him and he was sick, but he thought he would be able to get out of it. He didn't tell her, he didn't worry her–but he was wrong. He couldn't fix it, he couldn't stop what Finn had done to him. And so he died, Kol Mikaelson died after finally feeling like his family cared and that someone could love him. He died after getting out of the life that his Mother had forced onto him when she made them all into vampires and it nearly broke him even after his death.
But then he heard whispers. Davina was in the afterlife, though she wasn't dead. So he went looking for her, he found her and he did his best to keep her safe until she could get back. Then? Well then he had hope that he would see her again, and hope that he could be with her again. He knew where the cure was after all, tucked away nice and safe in a coffin where Elena Gilbert lay. It wouldn't do her any harm–she wasn't a vampire for that long after all.
And so if he was able to be brought back? He knew just how to get back to a normal witchy existence, hopefully with the brilliant and wonderful Davina Claire there with him for the ride.