How Anakin Skywalker became Darth Vader.
We all know, in Revenge Of The Sith, after Anakin Skywalker was defeated by Obi-Wan Kenobi, he was left to die on a planet full of burning lava. Until The Supreme Leader found him and took him in. Later we see that he is turned into a thing , More machine than flesh. He turned to the heaving breathing Darth Vader. Since then he completely accepted the Dark Force.
There are no spoilers if you watched the movie, but if you didn't then go away!
Reasons of Kylo Ren coming back like Darth Vader.
In The Force Awakens, Kylo Ren (son of Han and Leia Solo) was defeated by Rey (who was completely unaware of how to use The Force) which was pretty cool. Kylo Ren was not killed but, abandoned on the planet, which exploded, but Kylo is most probably supposed to get pretty messed up body from this explosion, like Anakin, and its most likely that he was saved by Snoke, and fixed like Darth Vader and completely target to kill his nemesis, Rey. This would even make Kylo feel more like his Grandpa Darth Vader.
Reasons of Kylo Ren not coming back like Darth Vader.
Well first of all, this idea has been used already once in this franchise and many fans might not like this, whereas many would like to watch Rey battle Snoke himself. Although we are not told how or when did Snoke become the supreme leader, also his powers are also a huge mystery. And Rey has to finish her training and will get more better in wielding the powers of The Force, as the Force is already strong in her, who knows how strong will she get. She might even get strong enough to battle Snoke in episode 8.