Mistake 1: Curtis (and Melody) did not call 911 when the now deceased perpetrator was trying to break into the house.
Mistake 2: Curtis confronted a much younger man without his firearm.
Mistake 3: Melody fired a warning shot. (That could mean she believed the guy who broke into the house was not threatening death or serious bodily injury to Curtis or to her. The proper thing to do when someone is trying to choke the life out of someone is to dispatch that person with the first shot fired.)
Mistake predating mistake 1- not being armed....