That Asaad has used chemical weapons indiscriminately against his own people on multiple occasions is beyond dispute - well that's not entirely true. Asaad, Putin, Corbyn and SBF will deny it of course.
The latest attack came from barrel bombs launched from a helicopter. The rebels have no access to helicopters.
Russia is a permanent member of the UN security council and as such has an obligation to take action against governments who use banned weapons. They veto all resolutions to that effect every single time.
International inspectors were kept away from the site of the latest attack by Asaad's security forces for almost a week after they arrived in Syria. By now all evidence of chlorine will likely be removed. They will in any case be unable to say where the attack came from, only whether scientific evidence of a chemical attack was detected.
The UK, USA and France did the right thing. It was almost certainly futile in the long term but it made a point that had to be made.