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The Bye Bye Man's motivation
His name sounds really, really stupid. He doesn't like when people make fun of it. That's why he kills anyone who focuses on his name a little too long. Alternatively, he chose the stupid name on purpose
There's a few people In-Universe who apparently find it amusing/stupid too, to the point that they tell their friends how amusing/stupid it is...thus giving the Bye Bye Man more potential victims since he operates on Speak of the Devil logic.