When dealing with large groups of people, there is always going to be at least one person whose views vary from the rest of the herd. Some may be the weaker members that are mistreated by the others. Others may have joined with certain ideals, only to see the group shift away from those precepts over time. Some may be in the group solely due to peer pressure. Regardless of the circumstance, no group is truly monolithic in it's views. This allows for some drama as the heroes, or villains, attempt to exploit the one outsider in the group to help them achieve their goals. A mistreated villain may show sympathy to the heroes and give them the means to escape a deadly situation. On the other hand, an overlooked hero may be tempted by the villains to join up with them where he would be "appreciated". In games, the presence of these characters allow for the players to come up with non-combat means of achieving their goals. If they can just find that one sympathetic ear in the crowd, they may be able to talk their way out of this one.
But what if a group was truly monolithic in it's views. Where there was total uniformity in belief and action? Where the concept of the individual is an anomaly, there is only the group. Suddenly you are truly faced with a truly frightening situation, confrontation with a group that cannot be negotiated with, cannot be reasoned with and cannot be be dealt with without conflict or surrender. This uniformity is the hive mind, named for the behavior of certain insects. The hive mind is frighteningly efficient, due to the fact that there are no individuals in the group who may have ulterior motives to hinder the group effort. The most famous example in fiction is the Borg Collective from Star Trek: The Next Generation, a group that seeks perfection by forcibly assimilating other species and their technology into the hive mind. Unsurprisingly, it was only with the introduction of the Borg Queen, an individual within the group, that the Borg went from frightening menace to cannon fodder for the cast of Voyager.
As a villain, the hive mind provides a disturbing picture. As previously mentioned, there are no individuals for the heroes to find a sympathetic ear. Killing a few members will only send the rest of the group after them in full force. Adaptation to a threat is swift, there is no discussion within to delay things. And perhaps the scariest part of it all, is that no matter what the heroes do, the enemy's numbers continue to grow as new members are born into, or assimilated by, the group as a whole.