Of course, there is no justification for such an act, not in Islam, not in the rights of a nation to pursue its freedom nor in the right of a family to avenge the death of its sons. There is no justification, but there is reason, and Russia would do well to understand those reasons.
Unfortunately, the reaction of Russia's leaders has been knee-jerk. Medvedev says there will be 'no compomise', which eliminates the possibility of Russia ever finding common ground with those who are attacking her. Putin promises to destroy the terrorists, as if retaliatory violence will bring a solution. Rogozin wants to launch some sort of a pogrom against 'ethnic criminality', a move which could light a tinderbox of pent-up ethnic hatred.
Let's assume for the moment (as most in Russia have) that the bombers were Chechen. Then they will have many reasons to hate Russia and want to hurt it. Their land has been occupied by Russian forces for over 200 years; they are a conquered people and it was never their choice to be part of Russia. Their culture, language and religion are under threat. Their sons and daughters are brutalised, raped and killed on a daily basis by the occupying forces in Grozny and the puppet goverenment they have set up.
The message to Moscow is this: nothing will be achieved by force except a continuing cycle of violence. Negotiate a solution which is a compromise between Russia's legitimate interests and Chechnya's legitimate interests. Withdraw your troops and allow the Chechens to police themselves. Support the moderates and limit your fight to the true extremists. Build economic and cultural relations. Keep your nose clean.
Given the amount of blood already shed, it will take decades to restore trust in the North Caucasus. That, however, is no excuse for not starting.