FSA is just another sectarian actor in Syria's conflict. It has had ample time to try to distinguish itself as a viable and better alternative and hasn't. All of those armed elements, along with the Assad dictatorship, have repeatedly shown little respect for civilian protections. This disregard is common in ethnic or sectarian conflicts, and that's the kind of conflict that is gripping Syria.
Weapons leakage from the FSA and other armed groups has been a problem long before now. Contrary to the idealistic myths put forth by its supporters, most notably Senator McCain, the FSA has never been very reliable. It has frequently shifted ties, including collaborating with the Al Qaeda-affiliated Nusra Front in the past. Its choice to partner with ISIS with full knowledge of ISIS's practices, goals, and attempted genocide against the Yazidis vividly illustrates that reality.