China is, contrary to what media might want you to believe, not a sinister empire of evil. They are, however, a culture with a strict requirement to save your face at all cost.

Following Hanlon's Razor, never assume malice when stupidity will suffice. IF, and that's a big if, this virus was somehow bio-engineered, the most probable explanation is that it escaped from the adjacent lab due to humans making mistakes. Now what do you do in that situation? A virus you're legitimately researching escapes from a state sponsored lab. You try to cover it up, of course, which isn't even hard because it might have developed in nearby bats anyway. That way, nobody looses their face and you can try to fight the virus with everyone else.

That is an easier and more logical explanation than some "bioweapon".