I think a solution would probably be to just have kitsune outright reject Malice due to some instinctive bullshit telling them "BAD NO EAT THIS OR U DIE AND AND GHFJEFEF FTAGHN".
I would be perfectly happy with this.
Kev wouldn't, since he wants to be completely immune to the literal embodiment of 'fuck this universe and everything in it, including you.'
And the reason they wouldn't go after it specifically?
It's impossible to properly destroy.
As long as there's negative energy around, it WILL find a way to manifest again, and it gets outright NASTIER every time it does. The reason there isn't an army of gods trying to kill Tor is probably twofold. 1) he's relatively sane, as far as collections of Malice go, and is safer than having it make four hellishly powerful false gods that go and beat up the four guardian deities of the directions again, and 2) the Headmaster doesn't like people killing his students. Like, at all. and has been proven capable of straight up writing people out of existence that piss him off. He's fully capable of doing it to Tor, too, which means that if he DOES go full nutjob, he'll die pretty much instantly.
The same reason why the Principal or vessels like Yukari haven't. It'd probably cause too much collateral damage, too much effort, and probably step on the toes of more than one other entity.
Far less effort to let the man control himself as he's MADE for it.
This too.
Killing him is absurdly hard and they don't know if he's capable of using Fusion yet. If he is, then shooting him with enough raw elemental power to blow up new jersey is precisely the WRONG way to kill him, since in theory this could trigger him turning into something WORSE. and even if he isn't, they'd basically have to raze the school, if not the city, to the ground to do it.
And again, he's a huge collection of Malice that
isn't actively trying to destroy the universe.