Short answer. Not necessarily. Under Article 11(3) of the Revised Penal Code, someone who defends a stranger from an attack does not incur criminal liability if there was unlawful aggression, the means used were reasonably necessary, and he was not moved by revenge, resentment, or other evil motive.
What the law says
do not incur any criminal liability
Revised Penal Code, Article 11 — Justifying Circumstances: Self-Defence And Defence Of Others. Read the full provision →
What the law says
Anyone who acts in defense of the person or rights of a stranger
Revised Penal Code, Article 11 — Justifying Circumstances: Self-Defence And Defence Of Others. Read the full provision →
What the law says
Unlawful aggression
Revised Penal Code, Article 11 — Justifying Circumstances: Self-Defence And Defence Of Others. Read the full provision →
What the law says
Reasonable necessity of the means employed to prevent or repel it
Revised Penal Code, Article 11 — Justifying Circumstances: Self-Defence And Defence Of Others. Read the full provision →
What the law says
be not induced by revenge, resentment, or other evil motive
Revised Penal Code, Article 11 — Justifying Circumstances: Self-Defence And Defence Of Others. Read the full provision →
Defense of a stranger is a justifying circumstance
Article 11 lists justifying circumstances — situations where a person do not incur any criminal liability because the act, though it causes harm, is regarded as lawful. Its third paragraph covers your situation: Anyone who acts in defense of the person or rights of a stranger, provided certain requisites are present. Because it is a justifying circumstance, a person who validly acts in defense of a stranger commits no crime at all — there is nothing to be excused, because the law treats the defensive act as rightful. But the justification stands only if each required element is present.
The three requisites
Paragraph 3 borrows the first two requisites of self-defense and adds one of its own. There must be Unlawful aggression — a real, imminent, unlawful attack on the stranger; without it, there is nothing to defend against. There must be Reasonable necessity of the means employed to prevent or repel it — your response must be proportionate to the danger, not excessive. And the defender must be not induced by revenge, resentment, or other evil motive. This last requisite replaces the "lack of provocation" element and guards against people who use a stranger's plight as a cover for settling their own scores.
Unlawful aggression is indispensable
Of the three, unlawful aggression is the one the whole defense rests on. If there was no actual or imminent unlawful attack on the stranger, the justification collapses no matter how well-meant your intervention was. Equally, the aggression must be coming from the person you hurt; you cannot claim defense of a stranger against someone who was not the aggressor. And once the aggression has clearly ended, continuing to strike is no longer defense but retaliation. The law protects the rescuer who meets a real, ongoing threat with a proportionate response — not one who joins a fight already over.
What can defeat the defense
Two things commonly sink a claim under paragraph 3. First, an excessive response: if the means you used were far out of proportion to the danger, the reasonable-necessity requisite fails, though an incomplete defense may still reduce liability rather than erase it. Second, a bad motive: if you were really acting out of revenge, resentment, or another evil motive, the law will not treat your act as defense of a stranger. Because the accused invokes this justification, he generally bears the burden of proving its elements, so the surrounding facts must genuinely support the claim.