A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 26 July 2007
I have a 2 Force Unit in battle. (2 Force total)
My opponent has a 3 Force Unit and a Dishonored 4 Force Unit. (7 Force total)
As an action, I play Unexpected Betrayal:
Battle: Target a dishonorable Personality in an army. Cards in that unit cannot perform actions. During battle resolution, the unit's Force is added to your army's total instead of its controller's.
My opponent passes. I pass.
Based on my reading of the card, during resolution my army total force is 6, and my opponent's is 3, thus I win the battle. Is the betraying unit destroyed? Can his force contribute to his own destruction?
Martin Lund (rules official), 26 July 2007
I have a 2 Force Unit in battle. (2 Force total)
My opponent has a 3 Force Unit and a Dishonored 4 Force Unit. (7 Force total)
As an action, I play Unexpected Betrayal:
Battle: Target a dishonorable Personality in an army. Cards in that unit cannot perform actions. During battle resolution, the unit's Force is added to your army's total instead of its controller's.
My opponent passes. I pass.
Based on my reading of the card, during resolution my army total force is 6, and my opponent's is 3, thus I win the battle. Is the betraying unit destroyed? Can his force contribute to his own destruction?
It plays out exactly how your read it from the card.
The Betraying unit can easily contribute to its own destruction due to the Force swing provided by Unexpected Betrayal.
Just remember that even if it contributes Force to your army, the Unexpected Betrayal says nothing about changing who controls the card or what side it is on. It is still part of the other player's Army, not yours. (And that's why it dies if that Army loses the fight).