Here's why:
* His motives: Prince Charming is driven by his desire for power and control. He wants to marry Fiona for her kingdom and doesn't genuinely care about her.
* His actions: Charming schemes to sabotage Shrek's relationship with Fiona and tries to harm Shrek in a fight.
* His personality: Charming is arrogant, manipulative, and selfish, not the charming and virtuous prince you'd expect from a fairytale.
While he might appear as a "prince," his actions and personality make him more of an antagonist in Shrek.