Question: What (if any) is the significance of the "OZ" graffiti that pops up throughout the film? It became quite distracting as I thought it would pay off at the end of the film.
Answer: The 'OZ' sprayer is a very disturbed man who claims to be an artist but the courts think otherwise. You can found his OZ (which he claims to be read OLI!) everywhere in Berlin and Hamburg. It has absolutely nothing to do with the movie but you can't film a wide open scene in Berlin without taping it.
Question: Why do they simply kill dozens of people in the movie, but with him it has to be something long and drawn out, like poison in the hospital or drugs in the parking garage?
Answer: Common movie, and literary, trope. The hero can never be taken down by a simple bullet in the back, otherwise there'd be no suspense, no story. It doesn't make sense, but it's more exciting this way.