This is not an argument of "reason" but agenda.
Shooting a pilot descending in a parachute has long been contrary to international law and conventions. The pistol carried by the descending pilot is insufficient to "wage war" with, but is a reasonable means of immediate self-defense from those who may first encounter the pilot on the ground with hostile intent - until lawful authorities arrive and capture the pilot in accordance with law and conventions.
By the way, many western air forces equip pilots with survival gear with which to cope with whatever conditions they find themselves in after a premature "landing". The pistol can be used and one of the tools for living off the land and protection until SAR or the local authorities arrive.
Now, as far as international law is concerned - where does Syria fit in? The Syrian government itself is guilty of numerous atrocities which would earn it a front row seat as the accused in an international court. ISIL has not exactly been a model of compliance with any law.
The Turkish act against the Russian airplane was probably not wise, but it was reasoned. This and other Russian airplanes have both been violating Turkish airspace in spite of adequate warnings and bombing Turkish allies, rather than ISIL, on the ground in Syria near the Turkish border. Remember that Turkey is adjacent to all this violence and would naturally wish to influence events in favor of their own security. It doesn't really matter where the SU-24 was when hit by the missile. The airplane had already crossed Turkish airspace more than once and can be classified as "unfriendly". The airspace was not exactly under Syria control at the time either, so claims of violating Syrian airspace sounds a bit strange considering all the US, French and other air activity over the country.