The source of her animosity towards Devon is made clear with the reveal that he was present, and partially responsible, for Isabela’s most shameful moment. When Isabela became a pirate she decided to enter the slave-trafficking business to pay off her debts. Devon hired her to transport slaves on her ship the Siren's Call. This reveal is shocking and provides future context for her intense dislike of slave trafficking. Unfortunately, halfway through the journey, they cross paths with an Orlesian ship - a group renowned for executing slave traders. When Isabela was unable to outmaneuver the Orlesians due to Devon surpassing the hold capacity, she took drastic action. The weight of all those people made it impossible to flee, so Isabela had her crew throw the slaves overboard, one by one, so they wouldn’t have the opportunity to mobilize, and she and Devon escaped.