Killian was in the army when he came home from duty in the summer of 2016. That was when he found a woman wounded on the street. When he approached her, she came at him and attacked him. He got bit by Amanda Delaine while she fed on him and he got turned into a vampire shortly after. He was then taken to the Millers for detox and to learn his new vampire habbits. He was in denial for a long time, which was eventually discovered to be because of PTSD he suffered while in the army. He made a recovery from that with the guidance of Jason Dawson who was a former soldier too. Jason used a lot of techniques to guide Killian into the world of magic. They have a good friendship because of this (see more : Jason and Killian).
The following 10 years he trained a lot. His former skills as a soldier proved valuable on missions and Killian quickly worked up his way in the vampire coven. Although Alyssa was resentfull at first (because his vampirism occured in similar fashion to how her mother used to recruit), she comes around after a while and allows Killian in the guard.
In 2030, Killian is sent out to go on a trip to accompany Sophia and Daphné's school camping excursion. He is ordered to keep a close eye on the two. One night, he goes hunting in the woods when he encounters a hostile figure in the woods. He is able to neutralize this person, but he can not prevent his accomplice to get close. Sophia ends up defending herself momentarly, and while Killian neutralizes the accomplice his guilt sets in. He introduces himself as the guard who failed to guard them, to which Sophia says he's the first guard who gave her a chance to show what she's worth. (see more : Killian and Sophia)