If both the killer and the victim were on foot with no car nearby to get in to and chase me, and I was in a car with time to stop and pick the victim up and shoot off into the distance away from the killer, then yeah I'd like to say I would do that. You can never say for sure what you'd do though because getting involved could be bad for you because the killer might recognise you then come to your house the next day and stab you in a revenge attack or something.
Or maybe I'm thinking too much into it lol...
I wouldn't do anything for sure if I was on foot though!!