Nobody is responsible for the actions of these goblins except themselves. You did your job---you went home, safe, to your family. You did everything right.
You might have been justified in shooting, but it might have taken time out of your life and money out of your life savings to get to that conclusion. It might have been a great deal more stress on you and your family to have gone through that.
And, worse yet, you said how quickly the guy turned to run; shooting him in the back without a direct threat to your life the moment you pulled the trigger might well have meant you would be the one ending up in jail.
Never mind the fact that you weren't going to get all three (four?) of them. So, maybe you could have shot one of the scumbags. The others were going to drive off anyway. Then what? They look you up for revenge. They go on committing crimes anyway and another innocent shows up in their cross-hairs even if you did open fire on the guy with the knife.
You are not the chlorine in the gene pool. You were not single-handedly going to end all their lives of crime.
Please keep just these two things in mind. (1) You went home safe. (2) These scumbags did not make you into a killer.
That's a successful day, no matter how you look at it. Now it's up to the police to do their jobs and track these guys down. Don't hold it on yourself if they can't do that, because it is not and never was your job to do it for them.
Well done, Bruce.