One explanation may not be enough so I will support here with two scenarios:
Assume Tom is a cop, Robert is a civilian defending himself, and Laura is an aggressive civilian wanting to kill.
Scenario A
Laura attacks Robert and hurts him. Robert defends himself and kills Laura. Cop Tom sees this and must follow this former rule by killing Robert. Robert dies because he had to defend himself from Laura.
Scenario B
Tom can abuse his cop and kill Robert even though Laura did not attack him. Then Tom will lie and tell everyone Robert was attacking him or Laura. The judgement sadly will be trusted to the abusing cop, Tom.