I am sure that at least some area some local commander of either Al Nusra or FSA have joined Isis, for various reasons, like being besieged by Assad's forces, r

I am sure that at least some area some local commander of either Al Nusra or FSA have joined Isis, for various reasons, like being besieged by Assad's forces, running out of munition, etc. This is also the messy part of civil war to survive alliances are fleeting. And so it is totally believable that some weapons given by the West to support FSA ended up in Isis hand willingly.