Why do you suppose the just-war theory bans weapons such as chemicals weapons? Are these not legitimate weapons, just as guns and bullets are?

Why do you suppose the just-war theory bans weapons such as chemicals weapons? Are these not legitimate weapons, just as guns and bullets are? The purpose, after all, is to kill the enemy. Why is poison gas not accepted by the civilized world as a legitimate weapon?

Vesilind, P. Aarne; Gunn, Alastair S.. Hold Paramount: The Engineer's Responsibility to Society (Activate Learning with these NEW titles from Engineering!) (Page 39). CL Engineering. Kindle Edition.