Sarin is a nasty chemical that destroys lives in unimaginable ways. Whether it's really worse than any number of conventional weapons is rather subjective in the long term. Still, nations all over the world seem to agree that eliminating chemicals like sarin make war a bit less horrific.
The inherently unpredictable nature of chemical attacks is one thing that makes them so awful. There's no such thing as a precision strike when it comes to sarin -- the attackers simply drop the gas in a place where they hope to do the most damage. That means civilian casualties are difficult to minimize. What's more, it means that the overall strategic and military pros of deploying sarin are low, especially when compared to the cons. It also means, frighteningly, that opposing forces may consider fighting back with chemical weapons of their own.
That's why the United Nations has banned sarin. And that's why so many countries have agreed not to use it in a fight. Rogue countries that insist on using "unfair" weapons such as sarin may find themselves subjected to retaliation in the form of economic sanctions or military strikes.
So long as stockpiles of sarin and other chemical weapons exist, the shadow of these weapons will darken modern civilization. Unfortunately, due to the easy chemistry behind sarin and the fact that terrorist organizations are willing to use any weapon they can get their hands on, it's likely that we'll read about sarin for years to come.