>Kazumasa Iwao: It's a great question! Until now, in the Pokémon series, as you said, we never depicted any Pokémon as being necessarily evil. In Necrozma's case, depending on your perspective, you may think it's evil, but in other point of view it may be not. In fact, there is a natural conflict with the characters that appear in Ultra Sun & Ultra Moon.
>Iwao: The URS come from another world, where they confronted Necrozma and tried to control it, but something went wrong and their world became dark. A similar threat approaches now Alola, so they came to Alola to investigate what happens and thats what they are doing when they encounter player's character.
Necrozma just wants his just revenge.