In Jainism, ahimsa (non-violence) is the highest principle. Killing another living being is forbidden, so you should not kill the killer.

What you should try to do instead:

• Use non-violent means first: shout for help, create space, block the weapon with whatever non-harmful method you can, and try to restrain or disable the weapon without causing fatal harm.
• Seek assistance from others and authorities to stop the act and protect the potential victim.
• If you are forced to act physically to protect a life, the aim should be to stop the violence with the least possible harm, not to kill. Your priority is to preserve life, not to take life.

Notes on tradition:

• Both Digambar and Shwetambar lineages uphold ahimsa as central. They may differ in practical nuances, but neither endorses killing as a first response. They emphasize minimizing harm and protecting life.

In short: do not kill; strive to prevent the murder through non-violent means, and only use the smallest possible force if absolutely necessary to save a life.