We are attempting to conduct high stakes diplomacy with Iran over its nuclear weapons program, in which Russia is clearly toying with the idea of backing Iran in a confrontation.
Dissuading them from doing so is important, to put it mildly, and Mr. Assad's regime offered a perfect opportunity, probably deliberately, to demonstrate that point.
In other words, why such an open massacre of opponents?
Partly it is to intimidate within Syia, but clearly another statement being made is,"Will the West dance, or just whistle Dixie."
One PGM strike at high value regime and military targets is a reasonable response function for the violation of Security Council Resolutions even the Russians had signed on with, and also a message: There are limits to Western patience,and if it's time to throw down, make our day. Otherwise, let's live a little longer and a little better, and deal with some obvious problems threatening the peace of the region, and the world, and in the case of the region,clearly the Syrian regime is a bad problem getting worse.