Savior of mankind

I don't know. It's clearly forbidden, but if it turns out in retrospect that the murdered person did commit murder, maybe he should be exonerated.
There is room to link this to the Reka's commentary on the Toss of Makot 5:1, according to which there is no obligation to die before the judgment is pronounced (i.e., the ruling of the High Court establishes the punishment and not merely reveals it). According to this, it seems that as long as the High Court has not pronounced its judgment, he is not obligated to die. It is true that some argue that here we are talking about the permission of the blood avenger and not about the obligation to die for the murderer himself.
I will point out that you did not do this, and it cannot be revealed that he is guilty of death retroactively, since if he dies in court, it will no longer be decided whether he murdered in the past or not. And perhaps this could happen if the blood avenger killed him during his trial before the verdict was finalized. But this will not end either, because the person is not judged in his absence.