@Destroyman
I'd agree with you most of the time, but in this very case it has been proven that said innocent Oni feels a strong compulsion to mass slaughter anyone that from another sapient humanoid race.
Up to the point that they get their horn lobbed off, the sight of a defenseless human baby would trigger in them the compulsion to tore that baby in half.
Oni feels the compulsion to exterminate humans no matter the circumstance, while they can fight it off for a time even the one who wants to get along usually ends up actually killing innocents just out of instinct (just like Mikoto's own father who killed his own wife when trying to keep the compulsion at bay, only to fail one day). And in many cases it's not just one or two people, but masses of people just by a single Oni (sometimes one oni can be enough to wipe out an entire city on a whim).
So while Mikoto is bad for not trying to find a solution he got a legit valid point in killing off Oni. As they stand as far as we know, any Oni will murder innocents regardless of intent at one point or another unless maimed(lobbing off their horns) or killed. Of course you could argue that he could go around removing Oni's horn everywhere, but then we have to consider that in a lot of cases, that is not something that is feasible in term of feasibility or difficulty and that in most cases that is just a straight delayed death sentence on the Oni regardless as their own kin will target them next time they meet.