Would have been more awesome had the killer met his fate before Johnson did.
Well, then he very well wouldn't have been the killer, would he? Or would that be the "horrible" doctrine of "preemption"?
This is not "Quantum Leap", "Superman" or "Star Trek", where one can time travel to the past...this is not "Minority Report"; there are no "precogs" to predict the future. Horrible things happen, and it sucks that they happen. Do we let it go, and risk it happening again?
Given the reality of an innocent's murder, and the choice of letting the murder live, or hunting them down and killing them, I say "let them die", "ready aim fire", "roll on two", flip the switch, pull the trapdoor, whatever...so long as the perp is gone.