1: a person who murders their brother or sister
2: the act of murdering your own brother or sister
3: fire that injures or kills an ally, or fellow countryman, commonly referred to as friendly fire
Many of our brothers in arms have been killed by fratricide, don't let you, or your battle buddy be the next one to be shot up by a jack legor a blue falcon.
Fratricide is when you kill a brother in arms (soldier or political/religous brother) who you are fighting with either on purpose or by accident.
In 9/11 the terrorists not only committed mass murder but they had also committed acts of fratricide against their own Islamic brothers.