News is coming through that the siege of the Russian School is coming to an end and near naked children fled as the assault progressed. According to Bloomberg, the storming occured as hostages began to break out.
I would be surprised if the SAS had not been offered to the Russians, but like the Kursk submarine tragedy of 2000 the Russians probably decided they could handle it. In any regard, if the storming was prompted as hostages were managing to escape, then there was no choice - there could have been many deaths.
At the time of writing, there are reports of 10 dead and 200 wounded, but this is unconfirmed.
For the life of me, I can't imagine what the hostage takers hoped to achieve. It's not going to bring sympathy to their cause - schoolchildren for goodness sake... and as a 'shock and awe' effort, it was too drawn out, given time for the hostage takers to be thwarted.