He should not have promised not to attack.
The Russian authorities did not plan to storm the school. There was a lot of chaos because the townspeople all had guns and wouldn't get out of the way.
People in the school think an explosion was set off by a terrorist by accident. When the kids ran, the terrorists shot at them. Then the Russians gave covering fire. They had no choice then.
Here is an article that says they did not plan to "storm" the school. A doctor named Roshal who has been involved in handling other terrorist attacks said nobody wanted to storm the school.
Everyone wanted to negotiate.
http://www.rambler.ru/db/news/msg.html?s=260005085&mid=5003793