Louis is captured by the Red Templars and was about to face certain death he wrote a confession letter admitting to the murder of his wife's brother, Garde, death. Once you find the letter your are face with the option. Give the letter to Louis to destroy or give the letter to his wife, Lynette.
Letter to Louis= Louis destroys letter, quest complete and Iron Bull slightly disapproves
Letter to Lynette= Dialog choice
"Louis killed your brother" = Blackwall slightly disapproves
"Things will be better now"= quest complete nothing happens