Not really perfect at all. But you are free to speculate. Supposedly, the killer of the cop was trying to get away. So fugitives don't do that..they just get away.
Round and round over and over..Because he was a cold blooded killer Oswald did it...Oswald did it because he was a cold blooded killer.
When did he start this murderous life? There is still no absolute proof that Oswald ever shot anybody.
Who could know for sure except Oswald? As a result, the question you asked necessitates speculation. Simply because we can't enter his mind to answer that question with certainty doesn't eliminate the evidence or create doubt of his guilt as you erroneously imply. But if you want speculation, Tippit is armed. If Oswald is not absolutely certain that he is dead or incapacitated, he still poses a potential danger. So Oswald finished him off. Better than getting shot in the back as he walks away.