Waiting 40 days is not a good idea. So much else can happen during that time. You need to strike immediately. There will be time for mourning later.
We never disregard this important time in the church. There are services all along from 3 days to 40 days. It would be completely inappropriate to seek revenge during this time.
Putin wants this handled by the government. He doesn't want everyone taking up guns. The Caucasus is a really ethicly diverse place. People will just go on the rampage and kill people based on ethnicity and not what crimes they have committed. It will spiral into genocide.
Russian experts have said that if this terrible carnage is used to justify vigilanteeism then we will have given the terrorists a big present. Thousands will die.