What all murderers share in common is a selfish disregard for the value and sanctity of life.  And isn't it all a matter of degree?  From a Stalin, Mao and Hitler to those believing they are doing God's (or Allah's) will to the Las Vegas shooter who apparently wanted notoriety to the Gacys and Dahlmers, and then to one who ends the life of an unborn human child only for the sake of convenience.  

Then there is Christ's declaration that a person who hates another has already committed a murder in his or her heart.  We can point in all sorts of directions to figure out why, but we need to look no further than the human heart, ours as well as theirs.