Remember that groups like "Al Qaeda" were referred to as "mujahideen" by the Regan Administration when they were fighting America's proxy war in Afghanistan during the 1980's - only at that time they were portrayed as noble "freedom fighters," engaged in heroic acts against the Soviets. The Pakistan secret police warned America that supporting Islamic groups fighting the Soviets should not be confused with them being pro-American - but the Regan Administration wouldn't listen and the US is still suffering the consequences.
In Syria, "Al Qaeda" may be ideologically opposed to the US but they're not stupid - executing a Catholic priest at the very time when Western powers are considering providing Assad's opponents with military arms is not only counterproductive, it would also incur the wrath of all the other dissident groups who desperately need that support.
If you ask yourself who has the most to gain from such an act, it leads right back to the Assad government!