50 year-old murder

Question: If David shoots Victor with intent to kill, and the attempt fails, and David is convicted of the failed attempt, and the malice evaporates, how do the following circumstances of the attempted murder still constitute proper murder without the mens rea at the time of incident?

You'd have to prove that the alleged murderer retained the mens rea over 50 years, wouldn't you? Is there case law that says mens rea is perpetual, and to erase it you must act opposite to the prior crime?

Seems pretty overkill to me.