Mutually assured destruction.
There's no path for Putin to use nuclear weapons that doesn't involve the utter annihilation of Russia. I would be willing to be that that will be a bright line for NATO, because once it's clear that Putin will use nuclear weapons when he's not getting what he wants, it's clear that there's no other choice that preserves independence other than retaliating with nuclear weapons.
Obviously the problem is that there are too many knives in China, and it's too easy for civilians to get knives! No one needs a knife; the only purpose of a knife is to cut and stab. The only solution is to completely ban all knives in China.
...Or they could seriously address the social issues that lead to certain segments of their population committing this kind of atrocity.
Hmmm. I wonder where else that could be applied...?