He let his irrational emotions control him. The creator of this topic ignores the fact that he:
Murdered dozens of innocent children at the Jedi Temple
Abused his wife
Murdered his own soldiers and officers
Murdered his former master Obi-Wan
Assisted the Emperor in leading his forces
Authorized the destruction of Aldeeran, and at least a billion lives
Despite the fact that he ended up murdering the Emperor, he spent most of his life in vengeful hate and thirst for power...
He is not even a legitimate candidate for a villain. Maybe, you're just a fan of Darth Vader or you're just immorally ignorant. Its just like saying I'm the villain for proving that families ate their own children as their Christmas roast after they slaughtered them for not believing in Christmas.
He might have brought the downfall of the jedi order, he might have betrayed them and yes, he joined the sith, but yet, what mattered most of him is that he sacrificed himself to kill Darth Sideous, to correct what he have done wrong. All heros have their own downfalls, like when Scarlet Witch and Quicksilver from Marvel turned their back on Ultron, they are known as heros. Then why can't Darth Vader, no I mean Anakin Skywalker be a hero too? Well, I don't know about you, but in my heart, he IS the chosen One. He did end the evil as the prophecy.
Anakin Skywalker spent his whole life protecting and avenging his family, only to lose everything standing up for what he believed in... Is that so evil? He loved his mother and vanquished those who killed her. He loved Padme and spent his teenage and adult life protecting her and attempting to save her. He stood for the Republic and destroyed any and all threats (including the Jedi because he saw them as a threat) to that institution. After his world collapsed and his wife died, children scattered, and his beloved institution fell, he clung to the only form of love he could find; the emporer. He did all he could to receive love and praise from the only person remaining in his life, no matter how "evil" others thought it was. It was still for his "family". When Luke returned to Anakin, he finally was set free from the shackles the emporer had on him and saved Luke by killing Darth Sidious and ending the empire. He was no villain, no evil doer. He was an affectionate man who would do anything for family, for love. He was Anakin Skywalker.