The story is that Darth Vader is starting to realize what an ass the Emperor is and that he was tricked into turning, but its too late to turn back and hes no longer strong enough to challenge Palpatine alone, so he makes an apprentice in secret to help him take control of the empire. Since the apprentice is secret he is instructed to leave no witnesses, thus why he is killing Imperials.
The game looks great, defiantly looking forward to it.
