Unifying ST theory

Snoke is ancient evil thing from Unknown Space found and unlocked or invited by Palpatine in some way .Palpatine has done this deliberately,as an ultimate revenge to the Galaxy that rejected his "visions".
Despite Snoke is alien,Palpatine arranged for Snoke to take control of Imperials after they retreat in the Unknown regions.

Snoke is enemy of life itself,he senses and influences Force sensitives from afar.So he did that to Ben Solo to, influencing balance of Darkness and Light within Ben,slowly turning him in to a serial killer and eventually patricidal monster.

Leia entrusted Ben to Luke, to try to change him.
Luke tried to train Ben, but eventualy he noticed something is wrong.

Luke's personal inquiry led him to finding of existance of Snoke's power and his insidious actions,of his unstoppable influence on Ben.

Leia and Han had their second child 10 years younger that died in a tragedy.Which was a ruse.

Luke,noticing everything is crumbling,loosing control of his Jedi project,took a desperate action.
Luke took Leia's child,and arranged that it looks she died, in the eyes of everyone else.Than he hide her on Jakku,blocking even child's memories.Jakku's force field generated from its unique core prevents any remote sensing by Force.,

After all that happened,Luke gone in to isolation, to find ways to oppose Snoke.
............
Kylo and Rey.Two halfs.
Rey is Ligt half, in her eyes is blue family light saber,preserved untouched,clean and pure.Protected by abandoning here on Jakku.
Kylo is ,altered , in his eyes is blue family light saber ,crossed by red ancient sword ,symbolizing Snoke's influence left in him.