Most of the "Willis is an asshole" stuff I heard actually came after he refused to do his regular cameo in Expendables 3 unless they paid him some huge amount of money for it. But that would have been in like 2014 I guess. Maybe he really was just going batty and people were just butthurt he was wanting as much money as possible...
I wouldn't consider Smith a reliable source either since I recall hearing he absolutely would lose his shit on people if they adlibbed in any way. I could easily see Willis just being like "fuck you, I say it my way" and that crybaby bitch holding a grudge over it.