Is complex villain necessity to write good heroic story

A novel with a Flat-Earth SJW Millennial automatically means that person is an antagonist of...common sense. 

No, ultimately stories about heroes are meant to about the hero. You don't need to fight a villain to death to be a hero, but rather do things for greater good (usually beating a villain falls here).

That said, doing a flat villain blows your story out of it's ass if the villain is the main focus. 

True, so in a way they are something that can straddle both sides.
If you think of something like Legend of the Galactic Heroes, someone like Reinhart is both a villain AND a pro/deutagnoist at the same time. 

Well if you give more details from both the protagonist and antagonist, readers will be more attached, then if it was only from the perspective of the hero.

Cause of simple logic

Every action has a reason behind it.

Even cruel and evil deeds.

If you could make the audience understand how the antagonist changed to evil and how the world he sees differs from how our hero sees then it is very thought provoking and gets the readers more attached to the story. 

Unnecessary.
Complex hero is necessary. 

Nothing overly complex. Revenge based or god complex is enough. If i want to read something that poked issues questioning human morality, i would just read the newspaper.
Thats why sometimes, a story just need a typical arrogant privileged young master villain with great backing. Easier and faster to digest. 

A complex character needed not to be sympathetic, but they often are, and their morality is usually in the grey zone, which did not fully come into conflict with the Heroic quality of the protagonist.

There're exceptions but most of the complex characters did not have the black morality. 

Good Heroic story shouldn't have a complex villain with grey morality. There must be a distinct difference between the force of good and the evil in the clash of ideology. A complex villain is the one in the grey zone, which blurred the line of heroic and evil.