If I remember correctly there was this theory that if Arceus is the God Pokemon, then Giratina’s the Devil. Well, if you think about it… Giratina’s Pokédex entry in Platinum mentions it being “banished for its violence”. Who or what could have banished it? Probably only its “brothers” Palkia and Dialga, or more likely Arceus, the God of the Pokéverse.
What’s more, it lives in its own little world, a world to which we’ve seen at least one person dragged for his evil ways. Sound familiar? Being a Ghost-type connects it rather closely to death, and its entry in Diamond/Pearl mentions it being found in an ancient cemetery. The evidence points to Giratina being The Grim Reaper, Satan, or both. Plus the existing role as the personification of antimatter, of course.
Oh, and did I mention that if you bring it back into the world of humans and normal Pokémon, from which it was banished, you enter Hell and bring its ruler out, giving it another chance to threaten the world…
Thanks to Appai for the corrections made.