Thinking about this lately, I am still somewhat unsure about what we should do with this situation. We still do not yet know who carried out the chemical attack, and a proper investigation is probably needed before we can decide on what to do. For all we know, Bashar al-Assad could've had nothing to do with it, or he could've been behind it. At this time it is unknown.
I used to be of the opinion that we should actually assist al-Assad in regaining control of his country again before literally double-crossing him and finding a way to deal with some of his brutal acts once the war as over. That way ISIS would be pretty much eliminated and the Syrian tensions could at least be constrained within the borders, allowing for a regime change should the leader step out of line. Not everyone would be pleased, but in the long-term, it could do some good and allow Syria to become a better place compared to the state it's in now.
Since then I am no longer of that opinion because while we the UK could've joined the Russians in keeping al-Assad, the Americans probably wouldn't have done so. Not to mention, the tensions with Russia wouldn't stop as soon as the war was over, because unlike the western powers, they seem to want to keep the current regime, whereas we seem to want to change it. I don't know. Perhaps some people could take that old opinion on board and build upon, or even tear it apart. I don't know, I actually am very unsure at the minute with my opinion on this matter, so maybe everyone else could help me out somehow with their views.