Gotta say, love when Kaoru comes up one way or another because people will break into "hate her" vs "kinda like her" warring camps.
It's the cheating. Check any manga with the 'cheating' tag, it's the same debate over and over.
Everybody wants to like the mcs, but nobody likes a cheater, so when they're the same people it's a given there's fireworks.
But does it even count when the soi-disant offender is quite open about the fact they don't do this whole "monogamy" thing in the first place? What you see is what you get, as it were, and people who elect to sleep in the rain don't have much of a basis to complain about catching colds.
We are not puritans from Salem, we shouldn't be policing people's thoughts.
Modern social media disagrees, as evidently does a nontrivial segment of this forum. Anyways, Mask definitely has a distinctly unhealthy combination of obsessive hero-worship and raging jealousy towards Onibi - the sort of complex that in an olden-time opera would presage insanity, murder and ruin ere the fat lady sings.
While that's definitely concerning - many people linked to Kaoru seem to be, one way or another - she's yet to do more than torture herself over it and seems to be quite conscious of just how dark the turns her mind sometimes takes are, so this hue and cry mostly reads as yet more bizarre moralism.
It's as if it came as a shattering revelation to some that people have flaws, often major ones, and so duly do their fictional depictions (indeed personal flaws are just about 101 Writing Interesting Characters)...