On one hand I feel it's a shame that he lost out on expanding his army, but on the other I wonder if they would be able to join up. All the other monsters had a curse that made them want to kill humans. The Shadow Monarch gains control of them after killing them, but he can't kill what's already dead. So maybe he wouldn't be able to take control of them. I hope that's true so he didn't lose out of a powerful army.