I remember reading that when the push was on to "liberate" Aleppo, that the Syrian oppostion had been pushed to the west and concentrated them in the region in and around Idlib, the sight of yesterday's gas attack.  At the time, politics dictated Syrian gov't forces win the day in Aleppo so as not to legitimize the Oppostions successes being realized against ISIS.  With the concentration of Opposition in Idlib, that would be very poor planning to have your chemical weapons stored in your base.  With that, I'd have to say Assad is responsible.