In our last session, there were, among the monsters, a defender and a vampire. The defender marked one of the PCs. After that, the vampire used an attack on this PC that commanded him to attack another PC.
The defender monster's mark gives -2 to attack rolls that does not include that monster, so the ruling was that this penalty applied to the attack from the first PC to the second one.
This makes sense according to the rules, but should it work that way? The way I see it, when a defender marks an enemy, they do that to prevent the enemy from attacking their allies, not to prevent them from attacking their enemies.