When someone responds to actions with an action that could result in death.
Often seen with Police officers. Officers have the right to use deadly force if a suspect has a weapon that can kill or incapacitate him. IE: guns, baseball bats, tasers, mace, knives, etc. Officers can justifiably use deadly force on suspects with knives and bats if they are acting hostile/are noncompliant, and are within about 25 ft. 25ft is the typical distance a person can suprise attack an officer before they can react and draw their weapon.
Tommy got shot when he came at an officer with a bat. The judge said it was a justifiable use of deadly force.