There is a pretty big difference between your example and the Tensing/Dubose situation. See:
Man A pushes Man B. Man B pushes him back.
Man A tries to leave. Man B shoots him in the head to stop him from leaving.
There are obviously situations where the actions of one person clearly caused the response from the other person. If Dubose had pulled a gun on Tensing, or acted like he was going to pull a gun, or tried to take Tensing's gun, or tried to run Tensing over, Dubose would be, at the very least, partially to blame. But Dubose didn't do any of that. He just tried to leave. That didn't cause anything.
Regarding the shoplifting example, you're clearly an idiot. I'm surprised your brain is able to power your hands well enough to put food in your mouth, let alone type.