That would be a horrible decision. Don't draw on the other guy if he is ingoring you, don't try to stop him. Your number one priority is to save the bleeding man. If the attacker acutally stops and complies with you that just makes things worse, because now you have to watch him instead of providing aid.
Now legally speaking you did not provide any information about where you live and how exactly are self defense laws written there but generally speaking if you shoot a fleeing criminal (even if you yourself saw the actual crime) you would get charged with murder and likely convicted in most places because self defense only applies while the threat to your or someone elses life is imminent. If the perpetrator is clearly fleeing you will not be able to argue that you were in danger.