WWII is not over between China and Japan.

Japan always has nuclear ambition, and even may have some nukes in their basement already. Because their war history was full of surprise attacks to all surrounding nations, China may choose pre-emptive nuclear strike instead of waiting for another Japs damage. Such a nuke-first strike is legitimate for several reasons. Technically, the WWII is not over between China and Japan, as the Japs have not fulfilled the Japanese Instrument of Surrender to China. The fact of WWII continues during which all available weapons have been used by the Japs, China has no obligation for not using nukes. More importantly, the Japanese declaration of nuclear power would automatically terminate the US nuclear umbrella, eliminating all complications for the Chinese attack.