What is the law called when you can kill someone when you think there threatening you?

The law you are referring to is often called "Stand Your Ground" or "Castle Doctrine" laws, depending on the context and jurisdiction. These laws allow individuals to use reasonable force, including deadly force, to defend themselves if they believe they are in imminent danger of death or serious harm. However, the specifics can vary significantly by state or country, including what constitutes a reasonable belief and the duty to retreat. It's important to understand that these laws are subject to legal interpretation and can lead to complex legal consequences.