It is interesting to note that the earliest forms of biological warfare were used in China and the Mid-East. The throwing of plague infected bodies over the walls of cities under seige was probably the first form of biological warfare. The second was the attempt by the Chinese to use Bubonic Plague as a biological weapon.
Certain "humanitarian" aid organizations in the 1800s sent smallpox infected blankets to the Indian tribes in the Western states primarily to exterminate them. Smallpox was a disease to which the Native Americans had no natural immunity. Many tribes were decimated as a result of these "gifts".
The greatest concern is will the weapon become a problem to the entire human population? Is the biological weapon something that can be controlled and contained? When I think of disease, mold, or blight; I think of a living organism that is capable of adapting and changing in such a way that it will survive long enough to reproduce.
The ethical problems of using biological warfare are that the civilian population is most likely to be affected by the use of biological weapons. Most non-combatants could care less about the political aims of the warring factions. The civilians want to have their patch of land, grow their crops, and live well enough to survive and go on about their business.