Assad wanted to eliminate any uncertainty about US intentions. The world’s perception that the US might eventually intervene was hurting Assad’s cause, who would be much better off if he could reveal that the US had no intention of ever intervening even if Assad used chemical weapons.
Using chemical weapons didn’t help Assad make gains on the battlefield. It didn’t signal to Syrians that he was willing to use these weapons — he’d already proved that he was willing to use them. What it did do was provide hard evidence that the United States was not coming to the defense of Syrian civilians under any conditions, and that the US would allow Assad to continue to fight to remain in power. Both of these signals would have the effect of undercutting support for the opposition.