Decapitation of the leadership is not an option from the Chinese perspective. It would lead to chaos, refugees, and the high probability of a reunification of the peninsula under SK rule (either that, or war with the US if they try to prevent it).
China would rather have an unstable NK as a buffer, than a unified Korea with a US ally and US military assets right on the border with China.
But a nutter with nukes who, as per comments above, might use them equally against China or the US. I suspect that the US and SK would rather have a demilitarised, China-occupied, NK rather than Little Phat Un building more and more nukes and delivery vehicles with the increasingly likely possibility that his delusions will drive him to use them.