Hadji is a term that is used in the Muslim world to describe an older man that has made the pilgrimage to the Mecca and is capable of wearing the black and white shemagh (head wrap) that denotes this.
The sticker with the slogan "kill hadji" is used by US Marines to mark who they are and what they've done. Warrior societies have marked belongings since the beginning of time denoting of the kills they have made and where they have fought. Even the Bible speaks of Sampson slaying his enemies with the jaw bone of a donkey and keeping thier foreskins in a bag to denote his kills.
Any warrior that goes to war and comes back will have his or her own way of dealing with the things they have seen and what they have had to do. Much more so when they were ordered, lawfully, by a sovereign nation to commit these acts.
My advice is not to criticize what you don't understand. Instead of calling Marines "fucking retards" or "backwood hicks", how about you walk up to them and say "thank you for doing the things in life that I neither have the intestinal fortitude nor the interest in doing". Because at the end of the day they did it for you and they asked for nothing in return. They walked through the valley of the shadow of death, they saw crushed and broken bodies burning in the streets, they pulled women and children close behind them to protect them when the enemy put them in front of them to protect themselves, and they returned home with no thanks, no answers, and a lifetime of trying to redefine themselves in a society that can't stand them or thier acts.
Who is posting this reply? A Marine infantrymen who walked in the lands of the Hadji and Mujahadeen and lived to tell about it
Semper Fidelis, and may the God who any of you believe in judge us all when the time comes, for I owe no man an explanation of my deeds and actions