I can think of a few notable people in history who fit that same "go through hell, turn evil" pattern. Adolf Hitler comes to mind. I could list many more examples, but it's hard to top Hitler.
I think it's also possible that Hitler suffered from some sort of mental disorder, though nothing I know of fits his behavioral patterns and war tactics other than the charisma and oratory skill he used to take over Germany. But he was irrational in terms of his tactics -- he was a brilliant politician, but he couldn't conduct a war if he tried, except by sheer force of numbers and delegation of tasks to generals. This is quite similar to Janus' apparent tactics -- he mostly foists military tasks on Ozzie. Of course, he doesn't really care about the war, which is remarkable considering how well he manages to execute it. As much as Janus wanted revenge on Lavos, however, I think even he would cringe or be angered at Hitler's atrocities. I don't know anyone that honestly thinks Hitler is anyone to look up to, and if I do, they aren't stupid enough to say it around massive me.
It seems like the dividing line between Janus and Hitler is that Janus is driven by an entirely different goal than that which he sets forth to the public -- the recovery of his sister and revenge against the patently evil Lavos. On the other hand, Hitler apparently believed he was doing God's work in eradicating the Jews. The history between him and his church, the Catholic Church, is rather interesting. Hitler actually conspired with the Church against German Jews in the 1930s to gain a majority vote in the Reichstag, the German parliamentary body. But later on, Hitler went on record stating that religion had to be removed for humanity to make progress. Perhaps he thought he was God. Perhaps not. Either way, Janus isn't that downright stupid. But Janus did believe in what he was doing, and yet he met failure at the end as well -- he was humiliated by Lavos in the Ocean Palace. They met roughly the same end to their quest, with the exception of Janus not quite dying. But it's interesting to see the slight differences in one's path that arise from motive, and it is in turn fascinating to trace that motive to its origins. Whatever disorder Janus had may have saved his life, and the history of the world with it, when he adopted new methods after the Ocean Palace debacle.
But, as far as public perception goes, barely anyone likes Hitler, while Janus is the most popular character in CT. I suppose that's the "glad it's not me, now shut up" complex kicking in.