Well, quick question. Do you believe "Assad is an evil dictator who gassed his own people"? If you do so, you're in the bottom quintile, quartile, third, half or bottom 80%. Syria never used chemical weapons and if they did so there is no proof of it.
Proof? No, but then you can only get proof in maths. Evidence, yes lots of it, and evidence of evidence being removed by russia and Syria. And evidence that they give contradictory accounts about the gassing. And those that were gassed. Believing that Assad, with help and backing by putin gassed his own people is taking the evidence and choosing the most probable. It might not have been him, but if it isn't then a long series of accidents and coincidences have occurred so it looks like him. Much more evidence that he did it than you would need to get a conviction.
Thus spake the master programmer: "After three days without programming, life becomes meaningless." -- Geoffrey James, "The Tao of Programming"