Warning: Realms of Runeterra spoilers ahead.
It's canon that mortals who show up on the Isles are often met by spirits of a similar disposition. The book says that "an innocent with a good heart...may be met by a spirit with a benevolent or empathetic nature" or "an individual filled with rage and anger will find themselves beset by screeching, howling wraiths."
Karthus was filled not with purity or anger, but hype. He was absolutely thrilled at the prospect of being undead. He came to the Isles all the way from Noxus for the sole purpose of dying there. So Karthus' first impression of the Shadow Isles was most likely a bunch of ghosts who were just as hype as he was.
And it doesn't stop there. Karthus' hype didn't just draw in the right kind of spirits to confirm his biases about undeath, it _made those biases true._ For him, at least.
His bio says that the "force of his desire to transcend mortality" was what prevented his personality from being annihilated. He got turned into a ghost but retained his sense of self (or most of it anyway) thanks to the power of his own hype.
And at the same time, the Isles' other spirits "were drawn to his passion like predators scenting blood in the water." So great was Karthus' hype that it registered on the senses of multiple beings, who despite being compared to predators at no point actually attacked him. From Karthus' perspective, being undead was an overwhelmingly positive experience because that's just the way undeath is, so he's going to turn everyone else. From the readers' perspective, being undead was an overwhelmingly positive experience for Karthus because he _enthusiastically believed that it would be_.
Conclusion: Hype (or "enthusiastic belief" if you want to be fancy) is so powerful a force on Runeterra that it can change a person's reaction to magic.