General Dunford identified one of the targets as a center for chemical and BIOLOGICAL weapons production.
Sarine and chlorine are horrible, but they pose relatively low levels of threat to American domestic safety. Biological weapons are vastly more dangerous.
Why did McCain actually go to Syria? What could possibly motivate an old man to do this? He could have communicated with every single person he met with remotely. He crossed the border from Turkey. Met with some Syrians for less than an hour. He was not collecting intelligence. He had diplomatic approval from John Kerry and the White House. He was making a delivery and it wasn't money or intelligence. There are far easier ways to move those securely. It was something physical. You can't search his diplomatic bags. Foreign intelligence agencies can't go after him directly without starting a war. He was a mule. He can't carry enough chemical weapons in his bags to matter, but virus or bacterial strain, all it takes is some cold storage.