Not sure I entirely agree with Benn. Yes bombs and guns have led to more deaths in Syria so far than has sarin.
Whether (Western) action in Syria is justified/called for now I dont know, and possibly no one can really answer that question in any meaningful way. But suggesting that there is no difference between sarin and ‘conventional’ weapons threatens to open the door to the use of chemical weapons again. And what about nuclear weapons… they are also just another way to kill people after all…
The effective ban on the use of chemical and nuclear weapons has to be seen as a major achievement of the last century. That people die from other weapons shows us what sorry state humanity is in, but should not be the indirect justification for the use of weapons we thought we had already banished to history.