I just don’t agree that a majority (rebels) are al-Qaeda and the bad guys,
SecState Kerry said testifying before the House Foreign Affairs Committee yesterday. I’m not sure which Intel reports he’s looking at but I haven’t ever seen it reported that AQ groups make up the majority of the rebels in Syria. In fact the opposite has been reported multiple times that they are a minority usually on the western side of Syria.
What is true is they are well supported financially, have organization, and most importantly well armed. With weapons you can control areas much easier and take areas from the moderates.
Being more violent they are able to take areas from the moderate rebels. This plays into the fear that they will have access to weapons we supply to the moderates.. Even if only by force they end up in the wrong hands.