You open yourself up to civil suit, and possible criminal charges if the restrained had you charged with kidnapping.

Probably best to allow the authorities to handle it, unless you yourself can articulate that you're in danger of serious bodily harm.

There's plenty of state laws pursuant to citizen's arrest powers, but federally, on a US registered Delta plane, it's a grey area.

A Good Samaritan Law is typically invoked with liability in a medical case. You're talking about forcibly restraining someone's freedom. You better be able to back up those actions.