Why Are Only Chemcial Weapons A Red Line?

Given the choice, how would you prefer to be killed? Whether you are blown to bits by a shell or choked to death by a chemical weapon, the end result is the same so why is the use of chemical weapons a red line but murder by bombs and bullets not?
The death toll in Syria is approximately 500,000 and the overwhelming majority of them have been caused by conventional weapons, a fraction killed by chemical weapons but why are our leaders only now concerned because 50 have been killed by a chemical bomb when we seemingly seemed okay with the other 499,950 who were torn apart by powerful explosives.
To make killing civilians with chemicals a 'red line' and the thing that triggers intervention implies that murdering innocents with chemicals is somehow worse than using bombs and bullets but the consequence of both remains the same, death or serious injury.
Some people may point to the suffering involved in death by chemical weapons but do you suffer any less if parts of your body are ripped away by a shell or missile?
The truth is that the fa miles of those killed don't care how their relatives died, they just know that they died and they won't feel any better about it knowing that if they had died by a bomb filled with chlorine the West will jump up and down and call it a red line crossed but if they had died by a bomb filled with explosives the West will consider it admissible and on the right side of their red line.