This change was likely made for a few reasons:
* Storytelling and Pace: The movie had to condense a lot of the book's plot and character development. By making Luke the son of Ares, the movie was able to establish his villainy and connection to the conflict earlier on, streamlining the narrative.
* Visual Appeal: Ares, the god of war, is a visually striking character with a powerful presence. This aligned with the movie's desire to portray Luke as a more physically imposing antagonist.
* Thematic Contrast: The film emphasized the contrast between Percy and Luke's character arcs. Percy, son of Poseidon, struggles with his power, while Luke, son of Ares, fully embraces his violent tendencies. This thematic contrast might have been considered more potent with Ares as his father.
However, this change sparked debate among fans, as it deviated from the original story. The movie's portrayal of Luke as the son of Ares has been criticized for:
* Missing Character Development: By removing his connection to Hermes and the original story's emphasis on betrayal and choice, the movie simplified Luke's character, reducing him to a one-dimensional villain.
* Loss of Nuance: Luke's original story explores themes of manipulation and the complexity of good and evil, which were diminished by the movie's straightforward portrayal.
* Disrespect for Source Material: Some fans felt that the change was disrespectful to Rick Riordan's carefully crafted world and character development.
Ultimately, the movie adaptation of "The Lightning Thief" made its own choices regarding Luke's parentage, for reasons that may be debatable but ultimately aimed to create a compelling cinematic experience.