I agree with wow. "Passive aggressive" describes the behavior of someone on the sidelines fomenting disturbances.

I also like the 3rd meaning of "virus" that Jackie posted.

Not as good, but in the general category--and certainly one who could manifest negative behaviors either passively or aggressively--is the spoilsport. Have we mentioned the spoilsport yet in this thread? It's not really what we're looking for, but I think it at least belongs in the discussion to be thrown away as, "No, that's not it at all!"

But passive aggressive? Yes, that's the behavior, by jingo! A passive aggressive virus! And there's no vaccine for viruses, right, Dr. Bill?