I’ve expressed my views on Assad’s chemical weapons specifically and previously on (chemical weapons generally).
Even if the recent awful events weren’t caused by Russian supported Syrian forces using chemical weapons – Boris quite carefully suggested absence of any evidence they weren’t – but were collateral damage due to ISIS / rebel or even circumstantial stockpiling locally, then Assad is still guilty.
Assad is responsible for the stockpiles being in his country and for agreeing to manage their disposal safe from use, deliberate or accidental. If there are Syrian chemical weapons unaccounted for, Assad needs to apply caution in any Syrian / Russian military action in his own country.