The White Walkers Built the Wall - Theory

[Heda]

I know some people like reading and some like watching a video, so here's a link to a video that explains everything typed out here.

We know the wall was supposedly built by Brandon the Builder with the help of Giants. No one really knows how he did it. The stories behind it is all from hearsay. GRRM said Brandon the Builder might be a mythical character that never existed. The Wall was built towards the end of first the Long Night, the war between the Children, The First Men, and the White Walkers. There is also the Horn of Winter, aka the Horn of Joramun, that can supposedly destroy the wall.

We know the White Walkers were supposedly defeated. So why build a wall to keep out something thats already being beaten? Why were some humans left north of the wall, along with the children?

It'd make sense that towards the end of the war, as the White Walkers were losing, they built the Wall to keep themselves from being destroyed, and keep the humans south of it from invading. It'd give them time to recoup and rebuild their army.

We know the WWs need humans to reproduce.  This might explain why the Black Gate (supposedly as old as the Wall itself) was made. The WWs could've waited outside it, slaying every human that passed through, adding them to their army.

This would be when the Horn of Winter was made. The humans had to bring down the wall to stop the WWs.  But instead, the Children decided they'd use a warding spell on it to keep the WWs from passing. At this point, the WWs abandoned the Wall hoping for another solution. 

The solution was a peace agreement between the WWs, the ruler of the North (Bran the Builder?), and the Children. Weaker humans (now wildlings, stronger after thousands of years of natural selection) were sent north of the wall to live so the WWs could reproduce easily. This created the Night's Watch to prevent them from going south again. They'd want to reproduce so they could continue their culture and not be wiped out.

The agreement probably also made peace with the WWs and the Children, but the Children wanted to be with the rest of the weirwood trees in the North.

This agreement ended the war.

Mance most likely broke another part of the agreement, which lead to the white walkers thirst for destroying all the wildlings, not caring about reproducing with them anymore.

What does this mean?

The white walkers mission could just be destroying the wildlings, and they have no interest in going south of the wall.  It could also mean that they are willing to make another peace agreement with the new Azor Ahai.

Why do they want Bran dead?

They could see Bran as a threat because Bran see's the WWs as pure evil and he knows a way to destroy them.

How would this affect the ending of the show/books?

The WWs could have another peace agreement, in return for Bran's execution. This could be part of the bittersweet ending GRRM has in mind.

TL;DR History is written by the victors. If the first men wanted to tell the story about Brandon the Builder building the wall, they very well could have to make themselves seem more badass, even though the White Walker's (Ice Monsters) built the (Ice) Wall. Bran is fucked.