From a Jain perspective, ahimsa (non-violence) is the highest value. Harming another person, even to stop a wrong, creates karmic bondage. So the ideal answer is: do not hit the killer.

What you should do instead:

• Try to stop the act without using violence yourself (intervene to shield the victim, block the killer’s path, or create a distraction).
• Call for help immediately and alert others.
• Use non-violent means to restrain or delay the situation as much as possible, prioritizing the safety of all living beings.

In Jain thought, the aim is to protect life while avoiding causing harm; true restraint minimizes violence and preserves intention (samyak sankalp). If you’re ever unsure, opt for the path of non-violence and seek help from others right away.