How can anyone justify such an horrendous attack on innocent people?

Amen to that. And pray that the response to it doesn't involve more horrendous attacks on innocent people.

Every atrocity is a reprisal, every reprisal involves another atrocity, and so it goes on for ever. And people say things like "The way you beat terrorists is to out terror them. And damn the consequences." People like todays suicide bombers.