citizen's arrest?

If a violent crime involving a lethal weapon was taking place and I was able to step in and stop the attacker and disarm and I had him under control. What would I do next? Should I put him under citizen's arrest and detain him until the police arrives? Or should I make sure the victim makes an escape and then bolt it out of there myself alerting the police once I'm in a safe area? It's unlikely situation, but it's one of those make the decision while my mind is clear rather than during the situation when I'm not going to be making the best of decisions things. So what would be your course of action?