The trouble with bets in this case is that the resolution requires a definitive answer which we will probably never get. If Spock were the judge and we gave him the details we can know and believe as "fact" he still couldn't say definitively who was responsible but I doubt he would bet that it was Assad's doing based on a logical deduction of cost-benefit analysis of who stood to gain by gassing civilians. 

 Assad is actually in an existential threat situation. He, his family, and his tribe can be expected to suffer horribly if the terrorist opposition wins control of Syria. I agree that he is probably as ruthless as his enemies. That is not the same as being stupid.

So now, with his battlefield situation looking far better than it has for years, Assad pulled out some chemicals he had hid away to do the one thing virtually guaranteed to cause him international condemnation, the one thing that very nearly got him bombed four years ago and may very well get the U.S. to attack him now? That would seem to me, and I think to Spock, to be quite stupid. But we all know that sometimes stupid people get in charge of countries so maybe so. For the terrorist rebels though, a gas attack that killed children and was successfully blamed on Assad would prompt the same results which would benefit them greatly. For them it would be smart even if grizzly. Unless one was to believe that they are not ruthless enough to do such a thing I believe that a bit of skepticism of the story so quickly taken as truth is in order. I bet Spock would agree.