What is the result of persuading Leliana to kill or not kill a traitor agent?
There aren't terribly direct consequences to this conversation with Leliana, nor does she have an approval rating like the combat followers. However, towards the end of the game
You get to help influence the choice of the new Divine, and Leliana is one of the candidates. If she does end up elected to the position, your choices while interacting with her will influence how the epilogue describes her reign. She may become a caring Divine who urges for unity, or a vicious and deadly Divine, described as having blood run through the halls of the Grand Cathedral.