With the recent chemical attacks in Syria and Trump's response, I wanted to explain in plain terms why chemical weapons are considered extremely heinous by the world community. I'm going to say up front, if you need a safe space, you don't need to be reading this post.
As far as my qualifications to talk about this, I have handled "war gasses." I can't tell you when, where or why, but I've been up close and personal with "The Big Stuff." First of all, CW's are not "gasses" like Oxygen and the like. They are really liquid chemicals atomized into a mist. For military forces, this stuff is considered a "harassing agent," as the enemy will sustain some casualties, however they have the equipment (MOPP gear, gas masks, etc.) to continue on with their mission. The forces will stop and don their gear, then continue on at a slower pace. To civilians who lack MOPP gear, chemical weapons are deadly.
Just to differentiate and give you some context, Pepper spray, tear gas and the like are non-lethal chemical agents. They won't kill you, but you will wish you could die. These work by irritating your eyes and sinuses.
There are three families of "war gas" chemical weapons, Blister, Blood and Nerve agents.
Blister agents work by causing internal and external blisters. Mustard Gas, which was used in WWI is an example of this agent. These droplets upon contact with open skin or inhaled, cause the affected tissue to blister up to 24 hours after exposure. These blisters are painful and are your classic first- and second degree chemical burns. When inhaled, the alveoli in your lungs develop blisters and you develop Pulmonary Edema. Your air passages swell shut and your lungs fill with fluid, preventing you from absorbing oxygen and you drown slowly over a day or two in your own fluids. Saddam Hussein likely used Mustard Gas against the Kurds in Halabja in 1988 during the Iran-Iraq war.
Blood agents are next up on the list. These are usually derived from either cyanide or arsenic. Phosgene Gas was used in WWI as a blood agent. The best known chemical of this in the civilian world is Hydrogen Cyanide. If a truck carrying this stuff ruptures on the highway, the police rope off an area about a mile wide so no one gets close enough to receive a fatal dose. This has to be inhaled because when blood agents come in contact with your red blood cells (which transport oxygen from your lungs throughout your body to the cells, then carbon dioxide back to the lungs to be exhaled) it saturates the red blood cells, preventing the blood cells from carrying out their function. You die within minutes, your body dying because it can't get oxygen. A gas mask can protect you, however civilians likely won't have the right kind of filter, and the right kind of filter have a very short time frame of use before they become saturated and ineffective.
Nerve agents are the big, evil, nasty bastards of this group. The well-known names of this class are Tabun, Sarin (used in Syria), VG and VX. These chemicals interrupt the transmission of signals going from nerve to nerve by disrupting the neurochemical process. Your muscles, which are no longer receiving control signals, activate all at once. Imagine a muscle spasm, occurring in every muscle in your body simultaneously. You collapse, every muscle hysterically tight (think trying to hold onto something that if you let go, you'll fall to your death and you know it). Oh, yeah, this includes your diaphragm, the muscle that makes your lungs inhale and exhale air. You inhale... and you can't exhale. You suffocate because you can't exhale (and then inhale again). Civilians have zero defense against this, because it can be inhaled or absorbed through the skin. It only takes minutes, if not seconds from initial exposure to death. The extra nasty thing about VG and VX-type chemical weapons is they have persistence. It's more of an oily gel that doesn't evaporate. When sprayed over an area, the contaminated surfaces can be fatal to the unprotected touch for days after it has been deployed.
CW's are evil, mean and nasty. While you can take some kind of cover from an artillery or air strike and have a fair chance of survival, these weapons will find you. You will most likely die quickly and relatively painlessly in an artillery or air strike. CW's let you know you're going to die and will deliver great pain and terror to you while you wait for the process to conclude.
When the leader of a country uses these kind of agents, especially against their own people, there is a special circle of Hell reserved for them, the one below child molesters. I will take months of screamingly painful torture before I deliver him there.