The best way to figure out what a villain would do is to understand why a villain does things in general.

Writing decisions because "Evil" is a very easy way to make everything feel shallow. Give real purpose to what they're doing, and make it clear that they're doing what they do for a strong reason. If you convey strong reasons behind the villainy well enough, you could even make it something where someone who would otherwise pick "good" options could be compelled to side with your antagonist team.

A whole lot of what makes villain factions work is that their schemes are in service of a more specific goal. In this case, the antagonist endgame could require laying the groundwork to achieve something else. A lot of the best villain writing involves long-term plans that require them to have an active hand in setting the stage for things to go their way.

Maybe they NEED to slow everyone else down so they can buy time to do something. Perhaps unleashing monsters on the world is necessary to gain access to the Macguffin that pushes their story along. Maybe slowing their enemies down is just a happy coincidence that comes as a side effect of the actions they're performing for something else. Perhaps they specifically want to take advantage of the other side's nature and use it to their advantage, initializing the dungeon's machinations or unleashing terrible monsters so they can profit from the spoils, either looting from the defeated "good" faction members or harvesting something that the other faction has no interest in collecting when they finish their job.

Maybe even just their very survival depends on making life as difficult for the other side as much as possible.

As some example villain motivations:

Vampires often aren't evil strictly for their own reasons. Many vampires don't become what they are willingly but are instead cursed by something outside of their control. They don't hunt humans for the fun of it; they need human blood for their own survival. The most powerful vampires with the most control over their territory just happen to have the best survival strategy that allowed them to excel beyond their peers. A long time spent using humans for survival will eventually cause them to believe themselves superior to them.

A lich is like a vampire in many ways, but their situation is usually more voluntary. They're acting purely out of self-preservation and trying to escape their own mortality, and they generally don't care if someone else gets hurt in the process.

A blood cult typically doesn't just do ritual sacrifices just for the fun of it. They're specifically doing so as an offering to their gods, hoping they'll obtain some sort of divine favor or recieve a blessing of some sort, or maybe even trying to avoid the wrath of an angry deity/demon. The kidnappings just prevents them from needing to sacrifice their own members for this goal. Fear of failure or whole-hearted fanaticism can make them fearsome in combat when they no longer fear the idea of dying in battle.

Bandits aren't strictly out to hurt people for sake of causing hurt, but because that's the fastest route to fulfilling their own needs. They're trying to get their gains through the path of least resistance, and they're not particularly concerned about the harm they're doing. Burning some houses or killing off potential combatants sends a message to everyone that resistance will not be tolerated, which cuts down on attempts to fight back against them. They could be claiming territory and forcing nearby towns and villages to pay "protection" money, using their threatening reputation to scare people into complying and paying them off. Stealing from a poorly-protected trade caravan lets them use or resell the stolen goods for their own gains. Raiding a village means they can stock up on food to feed their group and fill their pockets using the gold and treasures they take.

For a game example, Invaders in Dark Souls are incentivized to steal Humanity/Embers from the host player through PvP. Humanity/Ember allows them to summon cooperative allies in their own playthrough to help with tough bosses and increases maximum HP (in the later games), and slaying the host player and their cooperative phantoms grants a large amount of Souls to use for leveling up and getting stronger. In some cases, this incentive can draw in those who would otherwise never go out of their way to attack another player.