OSTERSUND, Sweden — When she boarded the plane, Elin Ersson had no intention of flying. Her goal was to ensure another passenger disembarked to avoid what she feared would be his certain death.

The man was an Afghan asylum seeker seated in the rear of the Turkish Airlines flight as it awaited departure from Gothenburg, Sweden, accompanied by officials assigned to transport him back to Kabul, Afghanistan.

In a 14-minute act of defiance on Monday, livestreamed via her mobile phone, Ms. Ersson refused to sit, delaying the departure. She withstood yelling by impatient passengers and crew members. She calmly responded: “I am doing what I can to save a person’s life.”

The authorities finally relented and took the man off the plane — a moment of vindication for Ms. Ersson, a Swedish college student and migrant-rights activist with rosy cheeks, oversize glasses and an earnest resolve.