You size up the situation and make the best decision, as ever. Wickedness can befall you at every turn, but that doesn't mean you should assume everything is out to harm you. You have judgement. Sometimes you mess up, and you have to fight your way out. Sometimes you don't. I hope your friend will be more cautious next time he stops to help a stranded driver (no matter how cute, how ugly, or how male they are). But I hope he won't stop helping people in need. We can't live our lives as cowards, and there is a price to be paid for that.
In the mean time, get more fighting skills. That's what they allow you to do: take on riskier situations and be safer. I take it that you don't know how to take someone down safely so he/she won't be able to sink a knife in your chest. There are people who can teach you to do that.
All I'm trying to say, is that people are saying what they would have done, and they weren't there.
Reading about something is different than experiencing it.
What you would do depends on hundreds of things that are too small to put into the story. Maybe even be able to articulate. You observe how people are, doesn't mean you can say exactly what those observations are.
And sitting at a computer, in a lit house, wide awake, is a lot different than:
Waking up to a crashing in the house, it's dark, and you're half asleep. And your family might be in danger.
I'm not saying the shooter is right or wrong. I'm trying to get people to look at things outside of the way they are looking. Look at situation from a different perspective.