This question contains spoilers of the Final watch (and previous books)!
In the Final watch (or Last watch) novel by Sergei Lukyanenko, the vampire
wants to kill Anton because he blames him for
death of his son Kostya
but at the same time, the vampire cooperates with the Inquisitor Edgar, who is equally to be blamed for it. After all, they chased Kostya alongside with Anton in the previous book.
Also, Anton didn't even kill Kostya, it was a suicide or rather an unfortunate accident, as we know from this answer. Edgar mentioned several times, that the vampire wants to kill Anton after their task in Scotland is done.
Why does the vampire want Anton's death so desperately? And why doesn't he hate Edgar equally?