The Bloody Baron killed his own wife, a Ravenclaw student named Helena Ravenclaw.
He was deeply in love with her, but she left him to pursue her own dreams. This drove him to a fit of rage, and he murdered her in a fit of jealousy. He was then condemned to roam Hogwarts for his crime, forever haunted by his guilt and the memory of his lost love.
It's important to note that while the Bloody Baron's story is a grim one, he is not a villain in the same way that other antagonists are. His tragic backstory and the deep regret he carries make him a somewhat sympathetic character.