The idea of a linked consciousness is interesting to me. A group of independent beings sharing the same mind and memories despite separate bodies. A hive mind in other words. It is a pretty versatile concept. They can be the horde of suicidally brave enemies, knowing that their individual deaths don't impact the whole. They can be an open organization of strange beings, often with seemingly oracular knowledge and an odd form of speech. They can be a secret society with inhuman powers of strategic coordination and shared knowledge. However, the idea that prompted this post is the concept of insanity and hive minds.
I was thinking about the morality of the Cthulhu mythos and how the amorality of the major beings in the setting is compared to the interactions between a human and an ant. For most people, stepping on an ant is not a moral or immoral action. You may not go out of your way to step on an ant, but you also may not even notice their presence. If they are, say, climbing on your counter, you may brush them off because you don't want them eating your food, or for sanitary reasons, or a myriad of other justifications. Then I thought about the Elder Things. These are sapient beings on the same scale as humans, but given the tone of the setting may share the same attitudes towards humans. This may be because they are so advanced that they see us in the same way we see chimpanzees, somewhat intelligent animals with the potential to be dangerous. They may also be responsible for the creation of vertebrate life on Earth and may view us in the same way we might view a defective toaster. Regardless of their particular morality and relationship to humans, it got me thinking about what might make a species view other sapient beings in the same amoral relationship as a man and an ant.
So from the mythos setting, I got on the train of thought about hive minds. If a group has a linked consciousness from birth, they lack any other frame of reference. When confronted with another group that lacks this capacity, they might see it as insane due to it's radically different interactions from individual to individual. It may also consider the the other group to be rather stupid, or at least forgetful, as details shared by one linked entity are unknown to all individuals in the other group. If communication took place, and the hive mind was able to understand the concept of a group without a linked consciousness, it might grow to see them as inferior life, stuck in one body, with one life span. Amplify this feeling, and you could see the same relationship as between humans and ants.
Another option for why a hive mind might view an unlinked group in an amoral fashion is, bringing this post back around to the first paragraph, a acute case of solipsism. Depending on how much individuality each part of the hive mind possessed, the idea might arise that non-linked minds simply don't exist. Another, related idea is that non-linked beings are P-Zombies (Beings that lack sentience and Qualila, but act and react as if they do) and thus physically real (unlike in the case of the solipsistic hive mind) but without a true mind. To build on that last idea, if the Hive Mind sees others as physically real, they may not even need to lack sentience or consciousness to be considered incomplete. The mere act of lacking a linked consciousness could mean the hive mind views others as mentally incomplete, lacking a major facet of what it means to be a thinking, reasoning entity. Anyways, to go back to the idea of a solipsistic hive mind, solipsism in the psychological sense seems to be linked to isolation. This seems contradictory at first glimpse, considering it is a consciousness made up of many parts, but remember, in the examples given, the hive mind is the only one of its kind, while it may be surrounded by other beings, those others are either a part of itself or unable to think and communicate on the same level as the hive mind.
You can play with other types of insanity or mental illness in hive minds as well. If the Hive mind has trouble grasping that others do not possess a linked mind, they might become paranoid, thinking that the individuality of others is a cruel trick by other linked consciousnesses. Even if they are able to intellectually grasp that others do not have the same mental faculties as them, they might become overly fearful or cautious due to the unpredictable nature of others compared to a large group consciousness. They might find the process of dealing with so many different minds and thoughts tiresome or not worth pursuing, because of the mass of personalities individual's possess.
A disclaimer for people looking to incorporate mental illness in their games, be it in the form of a hive mind or an individual. Be respectful. It may sound obvious, or patronizing, but mental illness is a serious issue with a lot of baggage attracted to it. Clear the subject matter with your group first, consider that they, or someone they know may suffer from mental illness. Even if that isn't the case, don't treat it lightly, do some research and use mental illness in a way that is true to life and, for goodness sake, don't use it as a source of humour. If the subject isn't fun or entertaining for you or your group, don't put it into your game. Sadly, some systems don't follow this advice, but do the best you can, given the framework presented to you.
As a closing note, I was originally going to skip the conceptual write up and present this as a faction/NPC/monster, but decided that it didn't really fit any of the settings posted here. I may put something up at a later date that uses this as a jumping off point, but we'll have to see.