Yeah… I don’t know about your monsters. But mine react the same way people generally do in various situations.

They fight when they think they can win. Especially when they have the advantage of numbers or positioning. They run away when they’re being beaten badly. They try to escape. They do everything they can as individuals in order to survive. But against the overwhelming horror of a slow and grisly end? They break. They might beg and plead for their lives. They might mentally snap and rush the players. But occasionally? After seeing his or her entire family clan/tribe/whatever being ruthlessly butchered in absolutely demented ways by laughing “adventurers”, and realizing they’re the last one left, and that the people killing them all won’t even consider listening to them beg for mercy, my monsters will slump down and terrifiedly whimper, “No. No- not like this.” And then they’ll use thier weapons on themselves. If there’s gunpowder or something nearby, they may attempt to bait them in and take the party with them, instead.

As what is essentially a small group of mercenaries calling themselves “Adventurers” simply because it sounds nicer, it really drives home how depraved the players are getting when the monsters are terrified of them to the point of self-termination.

Try it out. It might get them to realize what they’re becoming. My table is cool with darker and more realistic stuff like that. Yours might be a bunch of power-trippers. They might not be.