The West arent arming the terrorists, the West's influence states such as Saudi Arabia will happily arm them instead.
All rebels are united as they are predominantly Sunni, it doesnt matter about their political views, they arent killing each other, which means they are co-operating. The FSA cant win on their own, arming them will essentially filter into arming all rebels, this is why the rebels are classed as one group, FSA dont fight Al Nusra, they fight with Al Nusra.
This is why the west should stop this ever souring support.