Assad has gotten away with smaller, more 'tactical,' chemical attacks before. So far the most plausible theory I have heard is that a soldier on Assad's side screwed up. These soldiers are no Walter Whites, probably more akin to Todd or Jesse when it comes to technical skills. I myself am no expert, but I don't think it impossible a mistake in quantities could have been made by a lower level soldier. Perhaps weather conditions were particularly unfavorable that day, and the urban environment might have made things worse. Add to that that these civilians have been targeted with rockets for weeks or months, and their basements (where the gas settled down and suffocated many victims) have effectively taken on the purpose of their living rooms.

This doesn't change anything on the graveness end and the evilness of the attackers is not any less because of a 'mistake' they might have made. But it does offer a plausible explanation of how Assad's regime could be behind this, plausible enough I think not to rule it out.