Any individual or group that uses a banned weapon should be prosecuted. But I'd like to point out that whether you blow someone up or gas them, they can die a horrible death either way. 

It's lawful for armed forces to kill people- be they soldiers or civilians- with conventional munitions and weapons. Some munitions remain lethal for decades, killing and maiming innocent people long after the war has ended. Chemical weapons, however, usually disperse in a relatively short period of time.

The kinds of chemical weapons used so far in Syria are probably less damaging in effect than some conventional weapons that are still legal to use.