It doesn't matter how long ago it was, how he feels now, he still committed a crime and still should be punished for it.
That's like saying, should a serial killer who killed 10 women, but managed to hide from the police for 20 years but was then caught be prosecuted? He might have decided he regretted murdering those women. Of course he should, he still killed 10 innocent women, who possibly had children and probably had families, whether he regrets what he did or not, he still took lives and justice should be served.
Whether they regret it or not the fact is the committed genocide, and millions of innocent men women and children died just for their religion, or because they were disabled, or because they were artists or because, heaven forbid, they spoke out against the dictatorial regime and dared to have their own opinion.
Time doesn't change a crime, nor does it change the fact that the crime happened.
Everyone who was involved with the Nazi party and the genocide that was committed, committed a crime and should be prosecuted accordingly.