Not to add to the theories but a virus doesn't have to be lethal to be a weapon. If anything, MERS showed us that a more lethal virus is less dangerous globally as it burns itself out fast. To be an effective weapon the virus just has to cause economic disruption, or cause political changes, or impose some desired restrictions or limitations on opposing nations or the world economy in a way that is not as severe to the nation creating the virus. It should also be noted that this virus had the greatest impact on people that have been eating high carb, high sugar diets for a long time. Does that describe a set of demographics? But I am no expert in this area so certainly speaking out of turn.
I should also add the possibility that it doesn't have to be a government performing these actions. It could simply be one jaded semi-suicidal lab tech that is really mad at the world and realizes they are holding in their hands the power to F the world. Have people thrown their lives away for lesser causes? Or maybe even incompetence? Have lab techs ever mishandled dangerous material?