What is an 'Innocent'?

I've noticed that when Catholics want to appeal to the emotions with regard to abortion or similar topics they frequently use terms such as 'slaughter of the innocents' and other phrases including the word 'innocent'. 
I really don't get it.  If one's fundamental position is that all life is sacred, and it is only God who has the right to give and to take life, why should it matter whether the person is an 'innocent' or not?  Is it less bad to kill a person who has committed a sin?
It seems to me that so much of the Catholic way of thinking is based on emotion rather than reason, and yet many take great pride in stating that they embrace reason.  Conversely they accuse those who relinquish their faith of being guilty of pride.
This morning I was reading the blog of the Archdiocese of Washington.  To an outsider these people appear to be so wrapped up in their narrow view of life that they have lost touch with what it is to be truly human. In one thread they repeatedly assert that someone who has relinquished his faith must have done so out of ignorance, and they all say they will pray for him.  I guess that's a charitable thought, but do please keep your prayers for someone who appreciates them.