After all, Demon Slayer's story literally kicks off after the protagonist's entire family is killed and his sister gets turned into a demon. However, though Demon Slayer's vicious demons are irredeemable in most instances, many of them have equally tragic backstories. The audience and heroes alike cried alongside these foes as they spilled their guts about their sad upbringings.
This article contains spoilers from the Demon Slayer manga.
Akaza is, by far, Demon Slayer's most tragic villain. Anime-only fans only recognize him as the demon who killed off the fan favorite Flame Hashira, Rengoku. However, his backstory drove manga readers to tears. Some might even argue that he's Demon Slayer's best-written character.
As a human, Akaza grew up in a low-income neighborhood and struggled to make ends meet for his sickly father. He resorted to crime to get the money for his medication. This made rounds, and his dad committed suicide so he'd no longer burden Akaza, who was soon banned from the town. A dojo owner named Keizo took him in and promised to teach him martial arts if he'd look after his daughter, Koyuki. Later on, a dojo leader poisoned Koyuki. This sent Akaza into a blind, murderous rampage, killing everybody in the rivaling dojo.