It can be from any medium, whether video games, cartoons, comics, movies, etc. It can even be fan-made.

For me (Spoilers):

It's Doofenshmirtz, in my opinion. He faced cruel and emotionally abusive parents, mean-spirited bullies, constant loneliness, and consistent rejection despite how comical his tragic backstories are usually portrayed. It's still heart-breaking for him to go through and the fact that he's still nice and trivial, as well as a great dad, despite being an evil scientist makes him all the more sympathetic and his redemption in the end of the series all the more wholesome and well-earned.

7601605
As much as I love Starlight. I have to give the edge to Doofenshmirtz.

7601605
Think I'm gonna go with Arthur Fleck/Joker from the Joker film. While the Killing Joke version of the Joker had only one bad day, Arthur had many bad things happen to him in the span of more than one day (getting fired from his job, which was the only thing that made him happy, having his medication cut off because of city-wide budget cuts, getting harassed because of his pseudobulbar affect, being made fun of by his idol, finding out that his mother is partially responsible for his mental issues because she did nothing as her boyfriend abused her and Arthur).

I think Arthur sums up his turn to villainy pretty neatly with this quote:

I used to think my life was a tragedy. But now I realize it's a fucking comedy.

Oh, and other favourite villain origins of mine include Lotso, Darth Vader, Ardyn Izunia, and the aforementioned Killing Joke iteration of the Joker.

7601605
Mojo-Jojo. Always felt sympathetic to that freak of nature. He was just a clumsy lab assistant who wanted the Professor’s love. Then, Mojo’s greatest mistake ended up creating his sisters and made him super smart. Of course, the professor became too busy to notice Mojo, and didn’t even seem concerned when he finally ran away. It was like the Professor didn’t even notice.

If we're talking backstory in the literal sense, then Zeke Yeager's is probably my best pick...at least the one I can think of at the top of my head (do suggest a great villain to refresh my memory). Reveals a lot about his character, makes his motivations more understandable and has you sympathizing a little for him. Yet, it does not redeem him. In fact, it further solidifies that the path he's taken is wrong, because he's ironically furthering the hatred and suffering he fell victim to when he was young.

But again, this is if we're purely talking about a BACKSTORY. Zeke is far from the best villain.