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The way I interpreted things before Legends was that Giratina was banished for being overly violent in its job. Mainly based on how it violently drags Cyrus into the Distortion World to stop him from remaking the universe in Platinum, and how in the anime it attacks Dialga for fucking up its world from the damage done to the real world with the events of the 10th movie. An overzealous defender of space-time.
Of course Game Freak had to remove all doubt and ambiguity and go "nah it wanted to overthrow god lol" because they needed to give the bad guy a legendary to partner with.
