Alright, since there are quite a few Pure Evil Fallen Heroes that I can't put into a single poll, I'll just list out the candidates and leave everyone to type in the comments. The advantage of this is that now people can type in more than one Fallen Hero, considering the large amount of Fallen Heroes. Personally, I feel making a villain a true Fallen Hero is an effective way of giving them at least some depth, especially if we get to see them turn to evil on-screen. These specific types of villains feel more fleshed out than simply Charismatic villains (those who are only pretending to be nice rather than actually being nice and then turning evil for whatever the reason). Even if a Pure Evil Fallen Hero becomes a one-dimensional Hate Sink by the end of the story, we will at least get to see more of their backstory and how they came to be evil. These Fallen Heroes may also be given more personality while they are still heroes, which is why I find them to be a great way to make a villain more dynamic (especially PE or simply one-dimensional villains).
Alright, that was way longer than I anticipated. I could just have you go to the Fallen Heroes category to find your favourite(s), but I'll list them here anyways for convenience. Note that I do not include any PE's from Fake Tragic because they may not actually have been heroes in the past (i.e. Drago Bludyvist had his village destroyed, but we don't know if he did anything heroic, and even so, it does not excuse all the atrocities he commits throughout the whole film).
Here are the candidates:
Again, feel free to vote for more than one, and also feel free to give your reasons/opinions on these characters. If there was an Approved Pure Evil Fallen Hero that I missed, you can vote for them as well.