User blog comment:IamJakuhoRaikoben/Chapter 503: The Last Sight Seen, Review./@comment-5303512-20160926122525/@comment-5303512-20160926144740

Tell that to the Tartarus members who aren't all "Kill Zeref" and have stated they are devoted their creator. A species who believes in a creator and higher power are instinctively pron to be devoted to their creator. To take their words as laws and ways of existance, and if need be to kill, they would act as if it is second nature. Just like Abraham commanded by God to sacrifice his son, he would do so not out of fear or hesitation, but out of faith and love for God, but then God stopped him before it happened. So yes, devotion and instinct are not that far off. While the other demons are devoted with love for their creator to kill him, END appears to have hate driving him to kill Zeref.