Flamethrowers, despite their fearsome reputation, tend to kill through asphyxiation rather than burns. This is mostly down to their use as room-clearing weapons - the intense heat and fire sucks the oxygen out of any enclosed space and replaces it with carbon monoxide. The range of a flamethrower is on the short side and, as a result, use of flamethrowers in the open field was rare.

Using napalm as a method to clear out dense jungle and urban areas is a different scenario. The United States, in 1978, stopped using such weapons due to a combination of concerns about public perception and the battlefield effectiveness of incendiary weapons against professional mechanised armies. Flamethrowers, napalm and other incendiary weapons - when used against civilians - were banned in 1983. The United States only ratified this treaty some 25 years later in 2009 and with a significant "get out" clause.

Chemical weapons were being used, discussed and condemned before the First World War. The Hague Conventions of 1899 and 1907 attempted to ban and control a number of weapons that were emerging during the turn of the century (including "the Discharge of Projectiles and Explosives from Balloons"). During the War, these rules were ignored by combatants on all sides. It was only in 1928 that international law expressly forbade the use of chemical weapons. Violations did occur, notably by the Spanish (Morocco, '24), the Italian Empire (Ethiopia, '35-'36) and the Japanese Empire (China, '32-'45).

The fear of escalation and the potential horror of general civilian exposure prevented both Nazi Germany and the Allies from deploying chemical weapons as tactical or strategic weapons during the Second World War. Germany kept large stockpiles of nerve agents (sarin, tabun, soman) whereas the UK and the US prepared mustard gas. Gas masks were distributed to citizens of Britain's vulnerable urban areas - no gas attack was ever attempted and the gas masks were never needed.

Gas attacks were just that horrific. Military thinking changed a great deal after the First World War as a result of the truly Hellish conditions that soldiers endured on the Western Front. Shellshock psychologically scarred a generation; visible and unsettling injuries were all too common (a major impetus for the development of cosmetic surgery and prosthetics). Gas attacks inspired true terror - a silent murderous fog, agonisingly slow deaths, and poisons that pooled in the bottom of trenches.

There's also a practical consideration - early delivery systems were almost as dangerous for your own soldiers as they were for the enemy. Initially, gas was sprayed in the direction of the enemy trenches and you prayed the wind wouldn't change direction. Gas shells were developed later but they still carried a certain amount of risk during transport and handling. Military theory during the interbellum moved towards manoeuvre and rapid deployment; the tactical effectiveness of gas requires a static and heavily entrenched enemy, a situation broadly incompatible with mobile armies.

Research continued after the Second World War by the US/UK and the USSR. Britain's testing slowed during the 60s whereas American testing persisted into the early 70s. Agent Orange was used extensively during the Vietnam War as a defoliant and was designed to be non-harmful to humans, although long term health effects are suspected. Soviet research apparently continued right until the collapse of the USSR and, indeed, a chemical agent was used during the 2002 Moscow Theatre siege. Over 100 civilians died due to poisoning.

Chemical weapons fell out of favour because people - the military and the public - have become more aware of the indiscriminate damage and relative ineffectiveness of these weapons. The primary utility of chemical weapons, in Yemen ('63-'67), in Angola ('84-'88) and in Iraq ('88), has been that of terror. Since the 1940s, chemical weapons have only been deployed tactically on a large scale once as far as I'm aware - tens of thousands of Iranian soldiers were exposed to a variety of nerve agents during the Iran-Iraq War ('80-'88).

The source of the chemical weapons used by Iraq was something of an embarrassment. The Iraqi government developed their arsenal with assistance from Western nations. This, combined with terrorist attacks (Aum Shinrikyo, '95 / Ormancik, '94) and lethal accidents (Bari, '43 / Sverdlovsk, '79), precipitated the Chemical Weapons Convention which entered into force in 1997. Thousands of tons of nerve agents and weaponised toxins have been destroyed since with - officially - only the US and Russia still maintaining a stockpile.

Of course, I couldn't possibly comment on recent events in Syria.

Despite being nominally banned for over one hundred years, chemical weapons saw significant growth and use during the 20th Century. The reason incendiary weapons have not been banned in a specific military context is that they have a degree of usefulness in a way that poison gas simply does not. The Vietnam War is something of a microcosm here: flamethrowers were used with great success to clear out tunnels and hidden positions, whereas Agent Orange was not only ineffective, it destroyed entire ecosystems. Incendiary weapons are used to this day, in the form of thermobaric munitions, to attack fortified positions. They're cheaper, more effective, and (a bit) less controversial than chemical weapons.

Actually, tear gas is not legal in a war, as it's categorised as a chemical weapon under the laws of land warfare. As a result, its use in a crowd-control type situation by US troops in Iraq would require the highest level of permission prior to its deployment. It's not alone; for example, the hollow-point ammunition commonly used by civilian police or private citizens on a daily basis is also explicitly forbidden from use in war.

You are quite correct that there is some room for opinion as to just what can be considered a 'humane' vs 'inhumane' way to get killed in a war. Personally, I doubt any method is going to be particularly pleasant. However, not only are chemical weapons nasty and painful to die from, they also have the issue that they are impossible to seek cover from, cover massive swathes of land at once, and will kill animals and humans both with great efficiency, often being dangerous for some time after the weapons are used.

Interestingly, the first prohibition on chemical weapons dates back to 1675's Strasbourg Agreement.