Getting the drop on him has everything to do with it. Action is faster than reaction. If he saw me walking out the front door he could get his weapon in the ready position (assuming it is a gun) and shoot me before I can think about anything else. On the other hand if I sneak around back, I have the element of surprise, and could from behind cover and weapon pointed at him order him out of the vehicle and on to the ground.
I fully agree that a stereo is not worth taking someones life over, but I was thinking more of apprehending the criminal and holding him for the police to take when they arrive. But you have to consider that he might not cooperate when you are trying to apprehend him and you might be forced to use deadly force.