One of the most well known manga out there is a twelve volume series that goes by the name of Death Note. This Japanese-based series tackles a controversial ideology that has been a huge subject matter for generations; do criminals deserve to die for their crimes? The main protagonist Light Yagami is a representational figure for this ideology and uses a mythical item known as the Death Note to create this reality. Death Note's main reason for its popularity is its complex storyline, tackling of real world issues, and the battle of wits between Light and the detective mastermind who goes by the single initial of L. Despite being created back in 2003, Death Note still remains popular to this day, with adaptations, including even a loosely created Netflix movie and a musical passing along the basic storyline.
However, Death Note has a rather huge issue that despite its great popularity has drawn many potential readers away from it. The problem lies with the portrayal of Light Yagami, who to a lot of people is an extremely unlikable protagonist. He quickly after using the Death Note gains a massive egotistical mindset, thinking of himself as a god who will attack anyone, even non-criminals, who come in his way. As a result, Light creates a huge body count of innocent people, whose only mistake was crossing paves with him. It is made worse when the very likable character of L is knocked out of the story, who a lot of people preferred over Light. Although the manga series could still be enjoyed if you enjoy analyzing the psychological aspects of characters like myself, the dislike towards the villainous protagonist Light Yagami can be said to be a huge problem. This change of perspective not only exists as a problem potentially plaguing Death Note, but other stories as well.
The use of a villainous protagonist is potential problem due to the differences in morals the audience will almost always have with the main character. Unless they are only depicted as villains from society as in the case of Wicked, the villainous protagonist almost always sucks away the investment the audience has towards the story. The unforgivable misdeeds of the villain gives a bad taste in the mouths of almost all readers. Someone whose sole passion is to put kittens in a blender will not get the support of the audience, no matter how well the story ends up being.
This is always the case with villains whose motivations towards their actions are not very relatable. Characters like Voldemort from Harry Potter could not hold a story on their own if the only motivations behind their actions are because they were already crazy and they believe in the disgusting idea that Mudblood wizards (those who were born from non-wizard families) need to be wiped out. Those types of villains have trouble holding the story as a protagonist due to their creation being solely just to be a threat to the main hero in the original source material. Despite the potential thrill writing about a well known villain for their point of view might be, most villains unless completely fleshed out are going to fall flat.
The only way you will create a story in which there can be a villain as the sole main character is if they are extremely fleshed out with their motivations and have relatable characterization that is developed through understandable circumstances. The reason the entire TV series of Breaking Bad works with Walter White being the central character despite his actions being considered villainous is the depth inside of his character. Walter White became a drug lord out of a mixture of seeing others benefit from his own ideas, having his talents in chemistry be completely wasted with a lackluster job as a science teacher, and on top of it all getting lung cancer at fifty which would suck his family dry of money from treatment as well as from his potential death. He just did not decide after a night of heavy drinking to become the persona of Heisenberg. It became a reality slowly from the difficulties of his life, which dragged him into the drug business for the high amount of money and praise he could receive. Even after he is without a doubt a villain by the end of season two, audiences still adored the character due to the motivations that made him into one of the best written villains in TV history. Characters like Walter White are what make this trope work well, something a lot of writers miss when trying to write stories with the villain as the main protagonist.
Like the heroic variant of main characters, a villainous protagonist must be well fleshed out in order to work well in the storyline. Additionally, the audience must relate to the character in some way, whether it is their constant failures in the past or just being gutsy in even the most stressful situations. You cannot just write a villainous protagonist just for the heck of it! It would be like having a taco without the shell. The story is going to be lacking an essential element that would have otherwise made it enjoyable.