God sent Cain to kill Abel

Why did Cain kill Abel?  The simple answer is evil exists in this world.  The whole city of Newtown could have been inhabited by saints.  Evil exists, and catastrophic evil will attack good and bad people, sinners and saints, good pro-life cultures and bad pro-death cultures like ours.  A Christian response need not find the failures of the world to answer the first question.

If you asked these folks why Cain killed Abel, they'd tell you to punish the human race for the sins of Adam and Eve; God sent Cain to kill Abel. 

There is evil in this world which does not sleep.  It will be here until the end of time.  No law or protest will change that.  Evil conspires against us.  Evil dwells in us.  Even mental illness is a natural consequence to original sin.  We can only obtain perfect faculties at the recreation of the world, a new heaven and a new earth.

The line between good and evil is drawn not between nations or parties, but through every human heart. -Dostoevsky