The CHICOMs goal is military and economic hegemony in East Asia. A united Korea would be an economic rival but also a strong trading partner. It would also bring to an end US military presence there.
China wants NK as regime finished but in such a way as to minimize their risk of refugees and maximize the burden on SK. So I would read their actions through that prism.
I think China is going to use honey, rather than vinegar to pull South Korea (and ultimately a united Korea) into their sphere of influence.