if we don't clean it up - or in this case, intervene with threats until the threat of WMD is gone - a few bad things could (and most certainly would) happen.
Dude uses gas again. Monster flood of refugees into israel, turkey, jordan, iraq.
Gas prices skyrocket due to turmoil.
China, NKorea, and other nations decide to use WMD to quell rebels.... leading to great instability all over the globe. Imagine mexico using mustard gas, 50 miles from the AZ border, and millions of mexican families at the fence rioting to get into USA?
And worst of all, ANOTHER nation decides to step in the mess at Syria... and they decide to remove assad, put their own puppet in, and now you have China taking over things in the mid east. Bad longterm things here.
It's a global chessboard. There's NOTHING wrong with "just let them kill each other" but the ramifications are great - and it appears most of the people from both parties agree here. We dont need war - but we do need a guy not to gas civilians.