Demon Slayer: Kimetsu no Yaiba started off the way any great story does: with the brutal slaughter of an entire family comprised mostly of children. When Tanjiro Kamado returned home to find most of his family dead and his sister Nezuko turned into a demon, he made it his life's mission to track down the demon responsible.
Source：https://www.cbr.com/demon-slayer-why-did-muzan-kill-tanjiros-family/
