I mean, it makes sense why Saike's mad as hell. Johann, for as murderous and supremacist as he is, at least has a pitiable reason why he became that way and really doesn't hold much self-interest in his goals, thinking about what to do for other Oracle holders rather than strictly himself. Saike's been on that side of the fence before. And Saike himself selfishly plays the hero to his own detriment, slowly killing himself all the time by literally and directly killing himself, but he does it with good intents even if it messed with his own sense of self and morality and pissed off people like Kuroda who had his own reasons for what he did.
This "kid" on the other hand is just a Grade-A sociopath, who caused the entirety of the plot past Saike's beginnings as a means to keep doing what he has been doing for some unspecified amount of time. All of Johann's murders that Saike had to bear witness to, both prevented and not, all of the shit that has haunted Oracle users in general, and all of the suffering for basically everyone around him happened because this one dude just wants to be a kid genius with as many infinite redos as he could possibly ever want to feel special while controlling everyone around him for infinite wealth, non-consensual sex and getting anything he wants. And all of this latest shit, just for an object to let him keep doing it.
There isn't sentimentality. There isn't pity, not even a shred of decency and respect for anyone around him. Will just straight up doesn't care about the contents of the Akashic Records, which even means the entirety of whatever it could be used for, good or bad, means absolutely nothing the entire time. Entire lives died, bled and were dedicated to something he just sees as a method to removing his consequences and then starting again, like a child pretending all of his toys killed eachother and then he throws them away to do it with the next set. Evil might be pushing it from a greater perspective since Will is basically someone that never understood how to properly grow up when he indefinitely got his way, but it outright takes an opposite effect and steaming dump over everything that composes Saike as a person.