Why do people keep repeating this and treating it like its a universal truth? Apparently it originated in a joke book, even if some sage said it once doesn't mean it's frigging true. Have people not ever met people? There are endless depths of spite and malice in humans.
It's such a thought terminating cliché that people endlessly repeat, also used to excuse terrible behaviour.
Sorry the quote for people who don't know is:
"Never attribute to malice that which is adequately explained by stupidity."