At the point that the offender is intentionally trying to, or through grave indifference to human life, endangers your life and has to be stopped from doing so.
Ability. Opportunity. Jeopardy. The offender has to have the ability to harm you. He has to have the opportunity to harm you. He has to be placing you in jeopardy of harm, or making you think you are in harm's way. Once those three are met, it's game on. Example. He has an operable vehicle. You are in the path of his vehicle. He accelerates the vehicle towards you when he could have surrendered, or reversed away from you.