This question assumes that one should accept his punishment after being convicted of a capital offense.
Should one turn himself in to (Jewish) authorities after committing an offense for which he knows he will receive capital punishment? Assume that he would otherwise get away with the crime because the authorities would be unable to prosecute him: there was proper warning of two witnesses, but he was able to flee and is a fugitive.
Also assume that he would do tshuva and would not commit that crime again. Does turning oneself in constitute a violation of pikuach nefesh?
Obviously, halacha does not allow people to commit these capital offenses in the first place, but assume this question is bedieved, after he committed the capital offense.