Sometimes, other people can decide to put you in a situation where among all possible actions, physically stopping that person by any means is the least bad option by several orders of magnitude.

People refer to this as having "no choice", assuming most people understand "standing there and watching them repeatedly stab my infant with a fork" is not a choice anyone would make. It's an idiom.

I've intentionally left details vague — legally: a free man.

Tip: bare-handed, watch out for teeth — the will to live is strong.