O_o Um okay.
So, Vader invades Kashyyyk to hunt down a reported Jedi that had been living there among the Wookiees. He does so and then discovers the Jedi had a son. Instead of killing the child he decides to take it with him.
He raises the child as his own secret apprentice in the hopes of one day overthrowing the Emperor. To ensure that he puts the kid through some crazy savage training to bring out the highest Darkside Force potential. Because the Emperor is no joke.
Vader uses his Secret Apprentice to hunt down the remaining Jedi that escaped the purge. Eventually the Emperor finds out about the Apprentice, or he knew about him all along. Either way, he orders Vader to betray his Apprentice but to keep him alive so that he might escape.
Why? Because the Emperor knows with his training he'll want to seek his revenge. And to take on the Empire you'll need an army. So the ultimate goal is to let Starkiller draw out the hidden Rebel leaders that have lead pocket Rebellions so that they can be eliminated.
Of course the plan backfires, and those leaders that had been separate before unite and form the Rebel Alliance.
In his memory they adopt his family crest as the symbol of the Alliance:
He gave them Hope.