Murdering murderers makes one a murderer. If one is going to ignore any context and mitigating factors, no, murdering murderers does not result in net good.
E.g., people who have killed in self defense have been tagged as murderers. Murdering them is not at all a good thing (in my moral framework, at least).
E.g. #2, murdering one person for Reasons does not necessarily mean a murderer is going to murder again. So murdering them adds to the overall murder tally without necessarily preventing any additional murders.
There has to be some element of preventing future murders, not just retribution of past murders, for this to even be a debate, IMO. And then there's the bar of simply locking them up being insufficient to prevent them murdering in the future.
And how would one know with 100% certainly a person killed in self-defense despite being labeled a murderer by the applicable legal system?
It assumes an impossible perfect knowledge. Or if not perfect knowledge, some percentage of error, making the murderer of murderers guilty of occasionally murdering an innocent self-defense killer.