Colonel, did you ever run a "reverse" dungeon where the players had to play the monsters and the DM played the band of adventurers who ruthlessly hunted them down, invaded their lair, killed them and took their treasure?
I'm about to do just that to my group in a mini-campaign. Part One will be a hobgoblin chieftain, a bugbear, a shaman, a witchdoctor and assorted groups of goblinkind bowmen, slingers, and footmen -all given the goal of taking a tower manned by human men-at-arms.
Part two is where the typical band of adventurers comes to do to the goblinoids well, what they usually do. The monsters who kill the most of their enemies, loot the most from their victims and actually survive the assault from the human and demi-human party win the game. Yes, this will encourage the monsters to stab one another in the back as well as their foes.
One part of this is a change of pace. Instead of cooperation we have cutthroat competition. The other is for my own education as a DM, since I plan to take note of any clever defenses the players use against the forces of Good and keep them for future reference for this group and others. I've learned some of my best stuff from players, who always come up with things even the smartest DMs never thought of. Turnabout is fair play, after all.