Why don't you pose that question to the many religious zealots out there who slaughter, maim, disfigure, torture, mutilate, etc. in their gods' or prophets' name. Some even do it in the name of a state-sponsored religion known as patriotism.
It's the zealot mindset: It's their lovely way of "saving" by cleansing the world of all that's "unholy" to them. It's where "they're no good" is a key motivation.
Think about it.
It isn't supposed to make any sense to people who are rational, who don't wish harm upon others, and to pursue constructive solutions to life's problems. Take that ethic away, devalue human life, and place ALL faith in the hope of a divine reward after the dastardly deed and you can see the horror of it all. People who have no hope and feel that life is at a dead end are mere kindling on that bonfire. They literally go out with a bang and hope to take a few unwilling victims along with them to gain favor in that mythological afterlife.
That's why some people are creeped out by religion. They catch a glimpse of that big picture and thus a seed of doubt is sown.
All of that violent mess is based on nothing more that mythology thousands of years old which has very little, if any, basis in reality itself. People should be wary of religion and especially of those who deny the dark side of religion in general.