What is the most critical distinction between 'reasonable force' and 'excessive force' in self-defense law, beyond the attacker's initial actions?

The most critical distinction between 'reasonable force' and 'excessive force' in self-defense law, beyond the attacker's initial actions, is whether the force used was 'proportional to the perceived and ongoing threat' and 'ceased once the threat was neutralized'. Reasonable force is that which a reasonable person, under similar circumstances, would believe necessary to prevent imminent harm to themselves or others. Excessive force is any force used beyond what is necessary to stop the threat. Even if the initial use of force was justified, continuing to use force after the attacker is incapacitated, subdued, or has clearly retreated constitutes excessive force, regardless of the attacker's initial actions. The key is the continued proportionality and necessity of the force in response to the ongoing threat.