Certainly there is a lot to deconstruct regarding why the criminal wasn't down, one way or another, before the on-duty cop showed up.
I agree that there is a lot we don't know. The knife may not have been visible until he stabbed the guy. He also may have only just gotten to the front door when the police showed up.
Just thinking about the "What would I do in that situation" type analysis. What is the best course of action? What are the consequences for taking action, not taking action, or doing it early/late?