I'm planning to start a game of Rogue Trader with a number of new players that are new to RPGs. I've helped the players generate their characters and have given them a very very basic backstory to serve as a scaffolding for them to build.
One of my players is extremely enthusasitic and has come back with quite a lot of backstory — unfortunately, it seems like a lot of it will not fit the setting. For example, his character required a vendetta of some sort, so he wants his vendetta to be against the Emperor of Mankind himself. (I have yet to tell him that the Emperor is back on Holy Terra, ensconced on the Golden Throne, protected by thousands of psykers and that he will lead a very short life should he be found out...)
How do I go about making the appropriate changes without actually discouraging him from coming up with further ideas and feeling that he has lost control of his character's history?