They’re gullible or misinformed. Example: somebody who has been told the heroes are out to hurt them.
They are desperate for interaction, validation, kindness, or attention, and the dark side gives them those things.
They want to change their allegiance, but are pressured by people close to them to stay evil.
They have an otherwise noble goal that they will do literally anything to achieve. Example: somebody who wants to protect their child, even if it means throwing other children into danger.