Men have been killing other men since the advent of civilization. Not too surprisingly, countries at war are always looking for better and more efficient ways to kill or to destroy infrastructure during times of war, especially if those means of warfare will place their soldiers and equipment less in the line of fire. During World War I, the interminable trench warfare, led itself to the "solution" that instead of endless battles that never seemed to allow either side to gain ground, it was thought that chemical agents released into the air and placed in the direction of the enemy, would be able to "clear up" the trenches. This modified form of warfare was essentially taking previous chemical weaponry, such as poisoned arrows, and modernizing in such a massive way that properly applied it would either incapacitate for a long period of time the enemy, or kill him. From that perspective, chemical warfare was extremely effective; enemies suffering from the noxious gases were effectively incapacitated with blindness or severe lung damage or a painful death.
For whatever reasons, and there are several valid ones, chemical warfare was essentially banned by all civilized nations in the aftermath of World War I. There are plenty of valid arguments that make logical sense as to why chemical warfare was banned, such as the inhumanness of its effects, it also being an in-discriminatory weapon that kills or disables all life, whether animal, civilian, or military, and importantly since poison gases are subject to wind to which wind directions can be suddenly changed in ways not expected, chemical warfare fell quickly out of favor for warfare.
Yet, chemical weaponry is still used by most combative nations today, typically in the form of riot control using "tear gas". The reason why a chemical agent such as 2-Chlorobenzalmalononitrile (tear gas) is able to get a pass from the chemical weapons ban, is because it is perceived to be only an short-lived irritation which exhibits just a temporary disabling effect. However, tear gas is banned from international conflicts and is thereby only legal to be used in domestic disputes. This, in of itself, should be a warning flag that tear gas is not as innocuous as its name implies, which is true because tear gas incapacitates respiratory functions, can create skin lesions, severe vomiting, possible death, along with the implied tearing up and significant irritation to your eyes and consequently your vision.
While chemical warfare is banned and considered to be a crime against humanity, if used, indiscriminate bombing in which civilians and military targets are not properly segregated from the destructive force and wanton release of enemy bombing should be banned, and it is banned in most countries through Protocol I of the Geneva Convention of 1977, but this Protocol has not been ratified by the United States or Israel, along with a few other countries. This means that countries such as the United States can claim justification for indiscriminate "carpet bombing", which essentially is a callous disregard for civilian life and property, all for the nebulous gain of controlling or clearing land without proper regard for strategicneeds or humanity.
It makes no logical sense to on the one hand ban chemical warfare, while on the other hand allowing the indiscriminate bombing and its attendant destruction of infrastructure and population. The main difference between the two is that chemical warfare is something that most countries can ramp up to in a very short period of time, whereas aerial bombardment necessitates a multitude of tools, monies, experience, expertise, air support, and probably the purchase of said weaponry and training from a Western Nation.