Self-defense is one of the most common defense strategies in cases involving assault and other violent crimes. George Zimmerman used it when he faced charges of murdering Trayvon Martin. A police officer in Dallas is claiming self-defense in a controversial case after she shot and killed an unarmed man in his apartment.
News stories every day show alleged offenders going to trial claiming that they acted in self-defense when they shot, stabbed, or caused bodily harm to another person. Because in the heat of the moment, this is entirely plausible. Adrenaline may take over and you might defend yourself using physical violence.
Will this argument actually help you avoid penalties, though? That depends on the specifics of the situation.