Good and ethical people can become eviller over time.
Who has fallen the furthest?
(Living or dead, no fictional characters, sorry Anakin)
Bill Gates. Damnit Bill, stop helping people in Africa and fix your goddam OS!
Tsar Ivan IV was considered a benevolent reformer. But when his wife died, he certainly earned the name Ivan the Terrible.
Before he seized absolute power, Robert Mugabe was very highly thought of. So was Idi Amin.
A patriotic young corporal decorated for valour (Iron Cross, First Class).
Civil rights lawyer, opponent of Jim Crow laws in the '60s and proponent of the separation of church and state.
George Wallace could just as easily hitched his wagon to the star of desegregation back when he was a young judge entering politics.
I once wrote a sentence explain that Russia has a history of enlightened rule under reformers who considered the needs of the people and the nation’s future, with several names. I also wrote that there was an equal history of tyrannical despots who crushed everyone and everything in a ruthless quest for power… with the same three names. 
Wow.
Obama promised peace and prosperity and delivered torture and death with a pretty smile. And he’s still got a few years left to make it worse.
I always thought Nicolae Ceausescu was in that category. At the outset, his Romania was handed over to the Soviet occupiers, and he rejected them, insisting on going it alone, refused to let Soviet troops across the border, but of course had to conform with communist principles in order to be left alone. Things weren’t going too badly until an earthquake destroyed his oil refinery, and the economy took a nosedive and the Russians offered no aid. As the populace got angrier and angrier, the fist got tighter and tighter, and he wound up a paranoiac surrounded by enemies, and maintained a reign of terror pretty much in self defense.
Lots of good answers.
Did he necessarily change for the worse?
Saying nice things doesn’t make him a good person.
As for me, I’d like to nominate Robespierre. As a young man he hated the death penalty so much, he resigned from the bar. He valued life so much, he even protested against the war with Austria.
Decades later, after the French revolution he spearheaded the execution of tens of thousands of people, often for frivolous reasons.
I was recently listening to a podcast about Fritz Haber, who won the Nobel prize for synthesizing ammonia, used in fertilizer among other things. Apparently half the world’s current population depends on his ammonia method for producing fertilizer.
He later pioneered work weaponizing chlorine gas and other chemical weapons, before founding the Degesch Corporation, which later developed Zyklon-B to gas millions during the Holocaust. I don’t think he personally personally had much to do with Zyklon-B besides chairing the company responsible.
Jesus and/or Mohammed.
For 33 years /65 years respectively, not so bad.
For the next two millenia, they both have terrorized and divided the world.
I’m fairly sure Jesus hasn’t done anything to anyone for the last 1,980 years or so.
Good point. I’d say Obama was always an empty suit, willing to say whatever he thinks the crowd wants to hear. It’s always seemed to me that he’s mainly interested in the celebrity factor of being the president.
There are unfortunately numerous examples of distinguished scientists and physicians descending into quackery and/or lunacy. The most prominent victim of the “Nobel disease” who could be said to have fallen into evil is William Shockley, co-inventor of the transistor and Nobel Prize winner in physics for 1956. In his own words (1971):
""Babies too often get an unfair shake from a badly-loaded parental genetic dice cup. At the acme of unfairness are features of racial differences that my own research inescapably leads me to conclude exist: Nature has color-coded groups of individuals so that statistically reliable predictions of their adaptability to intellectually rewarding and effective lives can easily be made and profitably be used by the pragmatic man-in-the-street.”
He really thought Chinese were that much better than the rest of us? Wow, he really was far ahead of his time. 
You reminded me of the case of Jim Jones. I’m not sure he was ever entirely good, ethical, or sane, but he did start out working for racial integration and social justice.
Let’s just not turn this into a religious thread debate. If anyone wants to argue it, make a new thread, please.
Not saying this answer is not allowed, but it can have the potential to derail the topic, so this note is just a preemptive “Let’s not do that”.
I believe you misspelled “Romney” up there.
I came in to nominate Mugabe, and found that the second response nailed him. Gotta love the Dope. In the coffee group at work most folks have never even heard of Mugabe. Some have only the foggiest idea who Mandela was. These are people with at least a masters degree.
I literally cried with Joy in 1980 when listening to the BBC coverage of his victory. For the last twenty years at least, it’s been painful to contemplate that I once supported this guy. I have to keep reminding myself if the context in which he rose to power.
Fifteen years later I was weeping again listening to Mandela being sworn in, and I remember wondering if he would turn out to have feet of clay as well. Fortunately he never disappointed me.