There are two basic ways to look at banning weapons in war. The first, humanitarian view is that that war should be as "humane" as possible, limiting death only to intended targets, and delivered as quickly and painlessly as possible. With, of course, the possibility of rescue or recovery. Weapons banned in war inflict undue destruction on civilians.
The second view on illegal war weapons could be called the "Lee Perspective," so named for the Civil War general who said "It is well that war is so terrible - otherwise we would grow too fond of it." Since the Hague and Geneva conventions, we seem to have grown quite fond of war. The Sherman Argument would have it that putting rules on war turns it into a sport, instead of something to be avoided at all costs.
But no matter which side of that you come down on, the modern war does indeed have its rules. These are the nastiest, most horrible instruments of death and suffering that you won't see on today's battlefield - including a good number that might surprise you. These are the weapons that are not allowed, even in war.