why on earth do people have to be so stupid!

There is no sense trying to make sense of out things/events which are not sensible. In the same way that a psychiatrist does not attempt to enter the thought world of the insane, we ought not attempt to understand why such insane acts as these are committed against all humanity. What we can control is our response. I pray that it will be more intelligent, more rational, more considered, and more wise than the decisions of those who elected to kill innocents.