He killed a child to spare them of the fate of immortality as a monstrous child forever frozen in time. It’s like the unofficial rule of turning a person into vampire. NEVER CHILDREN. Anne Rice wrote how exactly terrible it is in “Interview with a Vampire”.
That’s cool and all but you realize that not every vampire story has the same set of rules lmao. There’s several vampire stories that don’t have the same universe rules. For all we know, the vampires could still grow in mentality despite their body not changing. (Like the vampire that calls the ML “brother” who looks like a kid, but likely has been alive for ages and ain’t exactly a child mentality). Regardless, he didn’t even give her a chance and immediately took her out.
From my understanding of this universe the ML and his brother are blood brothers and are “blue blood” vampires specifically meaning they were likely born vampires rather than turned into vampires so they likely age but much slower until they reach vampiric maturity. This could be entirely wrong but that’s how I understood the specific need to mention this specific exposition.
Ig they both kill kids. One kills mercilessly to eat and the other kills mercilessly for justice/humans(?)