laser on a defensive gun is worth its weight in gold in this circumstance.
every one knows (even though it is not true) that a laser shows exactly where the bullet is going to go.

You calmly pull out the gun with laser and put the laser CLOSE to the BG's eye. Then explain to him that if he lets her go RIGHT NOW he can live, if he hesitates he is dead.

but you have to follow through, and you must know where in relation to the laser the bullet is going to strike at that range. Then you do a simple offset aim with the laser and presto - dead BG, scared wife who is gonna kick your ass later... but she has to be alive to kick your ass!