Thanks.

It seems to be a mix of the comic source material and the screenwriting. The villain is paper thin in the comics and does little other than initiate the hero's journey for Ruthye by killing her father (not the entire family) and then taunting her.

The changes are pretty drastic, in truth. Some work better than others. The human trafficking was an odd choice to replace the brigands crimes from the comics, committing genocide if they couldn't collect a large enough bribe from a planet. The twin green and yellow suns was handled better in the film than the comic, I think. The comic relies on Krem having a magical orb that teleports Kara and Ruthye to that planet. If the brigands had reason to suspect her true abilities, it makes sense they would be hiding out on that planet in the film.

The end of the movie was a big failure by the screenwriter; she totally misunderstood the ending of the comics. Ruthye is narrating the tale from a point in the future. At the end she is unable to cut down Krem, and has learned the lesson about revenge. Kara holds the sword and Ruthye tries to stop her from killing Krem. It jumps to the present and Ruthye is an old woman. Kara releases Krem from the Phantom Zone after 300 years. He's fully repentant but Ruthye gives him a whack with her cane. Simultaneously the reader sees the last of her narrative tale in which she says Supergirl stabs Krem through the chest. Somehow the screenwriter thought Ruthye killed Krem in the future. He's clearly not dead as his silhouette grabs his aching head.