(Note: in some cases it's probably 'turning *more* evil made them less', actually.) Turning to some kind of dark side is usually associated with a boost in forbidden power, but in these cases, it also stripped away a part of them, or even most of who they really were.

* Anakin Skywalker -> Darth Vader, **Star Wars**: Anakin is a wisecracking ace pilot, inspiring (though sometimes concerning) to his comrades-in-arms, a Jedi knight, a lover, a warrior, and a good friend. But Darth Vader is (in the original movies) stiff and rigid in combat- seeming to go more for brute force than elegance. He's only a terror to his subordinates, never really a source of inspiration, and only at the very last does he seem to have any room in his heart for friendship, much less love.
* Davy Jones -> Still Davy Jones, **Pirates of the Caribbean**: Davy Jones was, Tia Dalma tells us, a man of the sea, and apparently many things but cruel. After the fickleness of the sea turned on him, and he neglected his duty as ferry of souls, he is hardly anything but cruel. He raids more brutally than most of the actual pirates we see in the film, taking no prisoners. He is also no longer quite a man of the sea- his *Flying Dutchman* can sail just fine underwater or through maelstroms, so he has little room for the use of any actual sailing skill. 
* "Neoth" -> the God-Emperor of Mankind, **Warhammer 40,000**: According to his acquaintance, a fellow immortal Perpetual, Erda, in the early days of mankind, the being at least once known as Neoth was "Funny. Witty. Articulate. Passionate," and driven to help. By the time he comes to reign in the 30,000s, though, he is even to his sons (and that tenuously; though they consider themselves his sons, he seems to barely consider them soldiers or subordinates, certainly not children) something of a cold and distant figure- to say nothing of how his subjects might see him. 
* Assorted original names -> Megatron, **Transformers**: There have been multiple iterations on just how Megatron came to be, but in most of them, he wasn't always a ruthless space warlord. The IDW comics say that before the revolution and ensuing war, he was a poet, and not a badly-regarded one. In Transformers One, he was a valued comrade to his fellow miners, but left them behind to become leader of the High Guard as the Decepticons. 
* Macbeth -> Macbeth, **Macbeth**: Macbeth at the start of the play is lauded for bravery, rallying the Scottish troops, dealing out blows to the enemy like cannons charged with double cracks, and so on and so forth. But by the end, after his regicide and killing of his friend Banquo, his troops desert under his lead and his own castle falls into the hands of the enemy. He's lost his commanding nature and boldness, leaving behind just the man who has the crown- and not for very long. 

Turning evil made them less than what they once were.