Einstien was the archetype of the Mad Scientist trope - certainly the reason why so many of them have crazy hair. Not sure there have ever been any truly evil scientists outside of Nazi Germany but then concepts of evil are a little relative (Einstien contributed to the Atom bomb, in terms of how much evil that created how does he compare to the Nazi scientists? Ok, unfair example because Einstein famously protested against its use but you can see the point...).
Scientists are rarely evil in the real world. They may come up with ideas which end up being used by corporations and governments for evil ends but I don't think many of them actually sit down and say 'I know, I'll come up with a way to kill more people in less time' usually they say 'Hmmm, if it were possible to split the atom there is a great potential for energy to be released by the resultant chain reaction...' followed by someone military saying 'Hey, yeah, and then you could kill a lot of people very quickly...'
If you want mad, though, look at Paracelsus. He was completely insane and incredibly arrogant (my wife, who is studying him at the moment, calls him a 'total cock' and I cannot disagree with her arguments because she is a much better historian than me
). May be a little early (because there was no real 'science' at that point but he certanly had the right personality. You might also want to look at Newton... the sane stuff he did with gravity was only his early work, he later went on to dabble in Alchemy which at the time was considered out of date and insane (ironically enough it was his work in optics which later allowed the atom to be split and there is a potential in nuclear physics to convert one element to another, i.e. lead into gold, but it does cost a lot more than it would to actually buy the gold in the first place...).