In the manga, was Galaxia just pure evil or was there a reason like in the anime?

In the anime, she was possessed by Chaos, but in the manga, she was simply manipulated by Chaos. So Galaxia had full control over her actions and she thought she could become the most powerful being if she stole Sailor Crystals and pitted Sailor Moon against Chaos in hopes they would destroy each other. However, this was a trick by Chaos and once Galaxia played her role, it turned on her.

Both Sailor Moon and Galaxia are the prime examples on how isolation and lack of support changes a person. They are tested on how they would react in the case of being alone but both reacted differently. Galaxia's loneliness gave her the desire to fill in the void. Anything that would make her forget her lack of sympathy and it was power that reached her. And so she let Chaos use her in the hopes of conquering the entire galaxy but her plan backfired on her. She knows she can't defeat such strong presence which is why she tried manipulating Sailor Moon's mentality in hopes that her power would burst and clash in with Chaos which would result in both being eradicated. In the end, she realized what she really longed for and it was Sailor Moon. It's not really her power but her hope and love she fought tooth and nail not to let go.

She was manipulated.  Or possessed like in the anime.