In their natural state, a Killer resembles a massive jellyfish, composed of hundreds of thousands of cells. It is not always easy to say where one Killer ends and another begins; they form new associations and break apart at will — in effect, they share both the advantages of being a hive mind and separate individuals. That said, without the constant turnover of cells, the mental processes of a Killer can become stuck in a rut, repetitive and uninspired. At the same time their cells, deprived of genetic exchange with unrelated cells, can become inbred, with all of the problems that can imply. The initial war in which they were involved encouraged isolation of individuals and small groups of Killers, so that both of these could easily occur. This tends to account for their seemingly irrational, monomaniacal and generally somewhat mad behaviour — they are trapped in a behavioural and genetic cul-de-sac from which they cannot easily escape. They are not, in a sense, truly aware of their danger. Despite their great size, individual Killers think faster than humans, with their cells communicating electrically via what is effectively a personal area network.
The Killers do have the occasional sociopath developing from their number, but evolution tends to weed them out. Killer sociopaths attempt to ‘eat’ their fellow Killers — in effect, rape them of their genetic memories rather than sharing — but any Killer that practiced this on a regular basis would wind up absorbing traits from the victim, including more stable thought patterns. The sociopaths are eventually cured through indulging their own desires; indeed, unlike humans, the Killers are generally unconcerned with such acts. Their immortality — the victim would live on in the victimiser — gives them a different perspective on such matters.
Once they had reached space, the Killers found that their technology rapidly accelerated, aided by their sudden access to infinite resources. They learned to extract and exploit raw materials from bodies in space (mainly using robotic systems) and, because of their agglomerated nature, it was very simple for them and their technology to effectively merge once it because sufficiently advanced. Unlike humanity, the Killers had no concerns about merging their minds with the technology; indeed, they found such concerns to be baffling. They were expanding their knowledge of the universe when they discovered, to their shock, spacecraft approaching from the inner system. The First Enemy had arrived.
What followed was a series of misunderstandings that led, rapidly and inevitably, to war. The Killers assumed that the First Enemy had been born in a gas giant, like themselves, and believed that establishing communications would be easy. The First Enemy believed the exact opposite. Unable to communicate effectively, war broke out when one side misinterpreted something the other did and spread rapidly across their system. For the first time, the Killers faced an enemy and it was a profound shock. They devoted their considerable intellect to building weapons and, eventually, bombarded the First Enemy and their homeworld out of existence. By the time the dust settled the Killers had been changed by their experience. The discovery that one of the rocky inner planets had given birth to the vermin/mites only hardened their resolve not to be attacked again. They expanded out further into space, broke apart several of the inner worlds for raw material, and learnt to harness the power of the black hole. Sublight colony ships were dispatched to nearby systems, followed by the first wormhole-capable ships. They discovered other forms of life, some on the verge of reaching into space themselves, and — believing them to be colonies of the First Enemy — destroyed them.
As they spread themselves though space, the Killers became more isolated and more focused on their goals, which had become guaranteeing the safety of the Killers by exterminating all forms of humanoid life. They didn’t realise that they were attacking other races — all humanoids looked alike to them — nor that some of the races simply lacked the technology to be a threat. Their odd sense of time meant that a threat was ‘now’ and had to be dealt with, even though the race they had discovered was barely capable of controlling fire. The more they spread out on their mission, the more isolated and monomaniacal the Killer Warriors became, incapable of accepting that they might be wrong, or that they were no longer part of Killer society. They were barely capable of adapting to new threats. This accounted for their apparent reluctance to destroy space habitats and other alien constructions; they didn’t realise that those installations could pose a long-term threat, nor were they capable of the imagination required to conceive of it.
By the time they encountered Earth, the Killer race had split up into several sub-sections. The Warriors, who lived isolated lives on their starships, the Builders, who produced massive constructions in space, and the Thinkers, who lived in the atmosphere of thousands of gas giants. The entire Killer race had effectively stagnated. They simply had no reason to innovate further. Worse, the combination of their odd time sense and their perfect memories act as a barrier to any Killer who might want to challenge the status quo; the facts that govern their relationships with other races are beyond question, there for the taking. The Killers do not keep secrets from each other. Indeed, they have little concept of the lie.
The Killers have no concept of sexual relationships, or sex at all. Killers breed by fission; splitting their cells into other cells that are generally part of the overall whole, or traded with other Killers. They don’t have to exchange genetic material with other Killers, although they do share memories on a regular basis. Pain is something that happens at a cell level, and even then only at a very basic level, so agglomerations as a whole do not feel it. They do feel unpleasant disruption if they lose too much of themselves in one go.
The Killer conception of government is bizarre, to human eyes. The Killers will quite happily argue for years, each Killer holding several different conversations at the same time, over anything they feel like arguing about. The sheer level of their technology and their general mindset means that they really have very little need of government, or even close cooperation within their planets. The best way to describe it would be to call it a direct democracy with heavy communist underpinnings, but even that fails to describe what it is.