Josef Mengele is one of history's most infamous Nazi war criminals. A doctor, he notoriously performed grotesque medical experiments on human beings without their consent. If you're strong of stomach, here's a small sample of what Dr. Mengele did to his victims, many of them children:
Twins were subjected to weekly examinations and measurements of their physical attributes by Mengele or one of his assistants.
Experiments performed by Mengele on twins included unnecessary
amputation of limbs, intentionally infecting one twin with typhus or
other diseases, and transfusing the blood of one twin into the other.
Many of the victims died while undergoing these procedures. After an experiment was over, the twins were sometimes killed and their bodies dissected.
Nyiszli recalled one occasion where Mengele personally killed fourteen
twins in one night via a chloroform injection to the heart. If one twin died of disease, Mengele killed the other so that comparative post-mortem reports could be prepared.
Mengele's experiments with eyes included attempts to change eye color
by injecting chemicals into the eyes of living subjects and killing
people with heterochromatic eyes so that the eyes could be removed and
sent to Berlin for study.
His experiments on dwarfs and people with physical abnormalities
included taking physical measurements, drawing blood, extracting healthy
teeth, and treatment with unnecessary drugs and X-rays.
Many of the victims were sent to the gas chambers after about two
weeks, and their skeletons were sent to Berlin for further study. Mengele sought out pregnant women, on whom he would perform experiments before sending them to the gas chambers. Witness Vera Alexander described how he sewed two Gypsy twins together back to back in an attempt to create conjoined twins. The children died of gangrene after several days of suffering. [footnotes omitted]
After World War II, Mengele escaped to South America. He died a free man in 1979. My hypothetical: Suppose, contrary to fact, that Mengele's experiments led straight to a cure for cancer. How should his obituary be revised?
In my view, barely at all. An obituary is a prime opportunity to fulfill Lord Acton's maxim to "suffer no man and no cause to escape the undying penalty which history has the power to inflict on wrong." A man like Mengele, who repeatedly tortured and murdered innocents, is beyond redemption. Any upside of his atrocities should be (a) mentioned only after enumerating his monstrous crimes, and (b) framed ironically. I.e., "Through a strange quirk of fate, the Nazi doctor who spend his career taking innocent lives indirectly saved millions."
So what? Most obituaries for world leaders take the opposite approach. Instead of vetting their records for capital crimes, and putting any such crimes front and center, the typical piece focuses on leader's "legacies." Thus, most obituaries for Deng Xiaoping emphasize that he dismantled Maoism and put China on the path of rapid development. Never mind the half century Deng spent in the Communist Party as Mao's loyal and murderous henchman. (And even if you discount this as Deng's misspent youth and middle age, what about his key role in the Tiananmen Square Massacre in 1989?!) I can see an obituary saying, "Deng was less evil than Mao, the greatest mass murderer of the 20th century." But to affirmatively praise Deng because, in his old age, he "only" killed people who defied his authority?
You could object that, by my standards, almost every world leader is a monster. That's not far from the truth. But either way, you've got to bite a bullet. You can condemn almost every world leader for their crimes - or condition your condemnation of Mengele on the medical usefulness of his experiments.