Question: What is the best tool that can be used to stop a madman from killing others?

To survive: Do not live in, or visit, places that ban honest citizens from having and using this tool for their own defense. If you do go, then you are knowingly increasing your risk level, so don't complain if something bad happens.

In the case of groups of children, responsible adults should be supervising things, and responsible adults generally have ready access to tools they may need.

People with objections to tools can put their trust in others to take care of the situation, and their safest course of action is to run and hide while things are being handled.