Bloodlust — D.L. Kramer

Mikhos knew the exact moment his life spiraled into hell: it was when the huddled old man in the alley attacked him. The infection that took hold in his body brought about changes that turned him into a monster, one that walked undetected with humanity. Now forced to kill several times a year just to survive, he only wants to be left alone, with only a few others knowing what he is.

Then Aleksander shows up and wants his help to build an army to make their kind dominant, creating a real dilemma for Mikhos. If he turns his back, Aleksander will continue with his plan and the human race will be in danger. If he joins him, he's positive at some point he and Aleksander will try to kill each other again, leaving a real mess.