I may have missed something here, but I don't see why he would even consider killing him - what motive does he have to do so?
He knows more than the protagonist would like, and the protagonist fears that he'll figure out the whole story and turn against him. The protagonist is especially concerned since he realized he's been underestimating Derik's intelligence, and he's not sure if he can control what Derik finds out about him.
If I'm asking for advice on a story idea, don't tell me it can't be done.
Sounds like it could go either way to tell the truth.
not knowing much about the protagonists personality it sounds like he could use Derik in the future there's a lot of momentum there for him to build up on: surprisingly intelligent, protective of friends, easy to manipulate, etc.
With some prompting he may even gain useful skills, although I have no idea if you want to go for something like Darth Derik type lieutenant situation.
Why can't he just mind control Derik like the others? And say that Derik finds out. Being a simple human, just how much damage he could do?
His mind control only works on vampires.
He's primarily worried about Derik telling others. Most vampires know that his mentor exists, but are unaware that he gave a group of children similar powers. Derik could potentially reveal to them that there are several children, all conceived at the same fertility clinic, who have these powers. Knowing that, it would be considerably easier for vampires to track these kids down and strike at them unaware.
If I'm asking for advice on a story idea, don't tell me it can't be done.
Hmmm... Derik already knows he's been deceived, what with their companion being a vampire and all, but still helped him. He sounds pretty loyal to me, and if he saved him once... If I were him, I'd let him live, I guess.
In the end, I think it depends on how paranoid the protagonist is. There's a chance (a small chance, as I see it) that Derik will create big problems. Would he be willing to risk that?
OK, this is a Villain Protagonist story. The protagonist is at least marginally on the side of good, since he kills non-friendly vampires, but he's possessed by a magic-eating spirit and only cares about the welfare of other possessed people. He killed his mother when he was little (though she was planning to do an exorcism, not realizing that would kill him). He's a Chessmaster, and will kill if it serves his interests, but he doesn't jump to Murder Is the Best Solution, either.
So, I'm not sure if he'll kill this one guy, Derik. Derik is his 'best friend', a mundane human with delinquent tendencies who he often uses in his schemes. Derik forced himself along on a trip with the protagonist and his MindControlled vampire minion, and the protagonist did his best to ensure that Derik was kept unaware of what was really going on. However, Derik's smarter than he thought, and figured out that their traveling companion was a vampire. When the climactic battle occurred, Derik revealed what he'd figured out, then intervened to protect the protagonist from his nemesis. He seems unaware that the protagonist is also supernatural, but there's a strong chance that he's guessed that or will soon. He also believes they were being held prisoner by the vampire, not realizing his friend was controlling their so-called captor. He probably won't be very happy with his friend if he realizes his friend made the vampire kidnap them as part of his elaborate plan.
So, does it make practical sense to kill Derik, or to let him live in the hopes that he'll be useful again in the future?
If I'm asking for advice on a story idea, don't tell me it can't be done.