All Minifigs consider themselves and, to a lesser extent, their immediate allies as 'the good guys'. Sometimes the boiling vat of psychotic violent tendancies and myopic insanity of a faction will tip over, resulting
in an army that revel in evil and doing everything in their power to propagate their image as truly unpleasant people. Nazis, Post-apocalyptic Mutants, The Cannibal Corps, Dark Lord Evil Blackheart Blood's Medieval Muthers and similar factions fit nicely into this stereotype.
Action-Heroes Clichés work just as well for 'bad guy' armies as regular ones, but as an alternative here is a list of Action-Villains Cliché examples:
Accent - Deep Voice / Over pronouncing
Example Action-Villain Feats: choke a minion to death to give all minifig allies +1 to all skill checks until start of their next turn, sever enemy hero's weapon hand, do a short range knockback attack on a target by psychically throwing nearby loose bricks at them.
Accent - ludicrously exagerrated german
Example Action-Villain Feats: fearsome reputation - give all minifig allies in line of sight -1 to all skill rolls and +1 to all damage/shove results until start of their next turn, locate the plot device, hilariously ineffective torture techniques.
More inspiration here
inhabitants, etc. Works best if it is an Evil Mirror of an enemy Hero.
Accent - posh version of mirrored Hero
Example Action-Villain Feats: produce unexpected hold-out weapon, infiltrate enemy ranks of mirrored Hero, sophisticated subtexts with mirror Hero.
Accent - Good ole Arnie
Example Action-Villain Feats: Inspiring charge - all minifigs and mounts gain +1 to hit and damage when charging the same group/unit/construct until start of their next turn, evicting an enemy from their horse and mounting it, crushing enemies, seeing them driven before them, and hearing the lamentation of the women.
Often seen leading cyborg or entirely mechanical groups.
Accent - monotone or massively pompous. Over-use of phrases such as "Excellent!" or "Destroy them!" are entirely appropriate, regardless of context.
Example Action-Villain Feats: Assimilation - enemy minifig is injected with nanites, plastered with cyberwear (replace one or more limbs with grey limb parts), or has their head replaced with a spare head of one of the Villain's army - it is now under control of the Villain's player until end of game; Machines Obey - move an enemy mechanical construct as you wish or fire a weapon; impotent rage.
Failure has similarly harsh results as Action-Hero Feats - a Dark Lord failing to chop off an enemy Hero's weapon hand will instead chop off their own, a Cyberleader failing to assimilate an enemy has just given them +2 armour but left them loyal to their own faction, etc. In short - imagine Marvin the Martian
attempting the same feat.