I thought it was all glaringly obvious. Anakin Skywalker, aka Darth Vader, said it best: "Together we can rule this galaxy, father and son." He might have added "...and daughter" because the whole story is one of the Skywalker family conniving to seize power, while trying to make it all look like coincidence, and pretending they didn't know they were all closely related.
The destruction of both Death Stars was orchestrated by Vader to discredit the Emperor in the eyes of the public, as the guy who had two white banthas destroyed almost immediately, thus paving the way for a palace coup in Imperial City. To this end, Vader had his daughter, Leia, convince her foster father Bail Organa and a bunch of his associates to set up the "Rebel Alliance" to provide a visible opposition to the Emperor. Vader captured Leia at Tatooine so that he could confer with her without arousing suspicion on the part of her "allies." In order to spring her, Vader played his hidden ace, his son Luke, who was mysteriously able to waltz into the Empire's new battle station, liberate the prize "prisoner," and waltz out with hardly any effort. Vader, of course, opened the necessary doors. This same Luke then led the attack on the first Death Star, but it's clear as day the battle station was rigged to blow from the inside, by Vader himself. As we know, Vader had never been especially impressed with the Death Star project to begin with, so its destruction was no big loss to him, especially after the Emperor had appointed Grand Moff Tarkin as the station's commanding officer, passing over Vader for the post.