Peasants should be delicious and tasty morsels.
Anyway, here is the idea.
Perhaps units could have an upkeep beyond the standard food/gold/mana. Example, if you have a hero who is a vampire lord, he could have a 15% chance each turn of eating a friendly alive soldier in the stack with him, or peasants if he is in a town, and each turn it increases by a 5% chance till he eats someone, then it resets to 15%.
And if you station him totally alone so he has no victims and he can't feed when he wants to, he gets pissed at you. Too many missed feedings and he leaves you. Just a little something to make the evil monsters seem evil and monstrous. Plus he creates a very real burden for having those top tier dangerous units.
Going the other direction, Angels could demand that your troops spend their time praying instead of training. It will keep the angel happy, but is it worth the loss of skill in your troops?
Basically, i want the top tier units to have personalities, not just be a collection of
stats. Having a demon that demands a unicorn burger after every fight, which forces you to keep unicorns on hand is a lot more interesting that a demon that is 32 attack, 28 def, 18 HP with 4 mp/turn upkeep.