I'm sure there were a few villains who were serious, but most of them were villains who were considerably weaker. When the Villain was stronger he toyed around with the Heroes which ultimately ended in his defeat.
I guess when you know you're stronger you'd want to toy around with your prey though.
With Frieza, he thought a Saiyan would never be able to stand up to him, hence why he toyed with Goku and then tried to make him realize how weak and useless he was by killing his friends which only served to fuel the fires of rage, resulting in the very thing he feared.
With Cell, I honestly don't know why the main characters didn't gang up on him. I mean come on, if all of them just went all out... or if Vegeta didn't get Cell absorb 18 then he'd have been killed easy.
And Cell, like Frieza, thought it smart to mess with the heroes emotions, resulting in the Ascended Super Saiyan (SSJ2) and ultimately his demise.
I think Buu was one of the only Z villains to be serious. Good Buu doesn't count, as Kid Buu is his true form.
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