It's because that darn Sixth Commandment keeps getting in the way.
'Thou Shalt Not Kill' can be a real impediment when you have scruples.
The original Hebrew translates closer to "Thou shalt not commit Murder". Killing in the name of God was just peachy as long as you had orders from one of His Angels.
Kind of a side argument though.
They WILL come to kill you if you don't follow their arbitrary, unConstitutional edicts. Does this seem fair to you?