If Assad's army were the ones who used it, does that justify military intervention?
NO.
And it has NOT been confirmed Assad used Sarin, just that it was Sarin gas.
The intercepted phone call indicates that those highest up in the Assad government were panicked to find out what had happened but we reported it only as a fact that Assad knew about it.
A few months ago the UN thought it looked like the Rebels were the ones who launched an earlier Sarin attack that killed less people.
In 2012 Rebels release videos showing they have chemical weapon capabilities and demonstrating them on live subjects.
Assad was not losing the war so he had no incentive to launch a chemical attack now. But who does have incentive? Not Assad...
But even if they did, we still have no business in this civil war.