That's assuming, for the sake of argument, of course that a biolab creating these types of viruses for "offensive" purposes would only focus on one, extremely lethal virus and nothing else. It's easy to imagine that less lethal viruses are also created to study the effects of different modifications, and these viruses may be kept under less strict supervision than the worst ones (increasing the risk of it spreading).

Anyway, there's no evidence so far to support any of these theories, but considering China's record I guess it wouldn't surprise me. For now I still think the easiest explanation is the most likely.