I would wager that there was already a conflict between the civilization those 3 were affiliated with and the kid prior to the altercation with Killy.

WARNING WARNING WARNING EXCESSIVELY LONG POST

Those three armed people were obviously soldiers and they had been fighting the Authority for quite some time as they say. The chapter after shows many people wearing the same uniform and has a very 'war story' feel to it. Killy was lucky that those three happened to be merciful in the end, even though their distraction with him might have been a factor in their deaths. It is clear that they had never heard of people coming from below and so his presence there was extremely fishy, especially considering that their enemy could look quite human.

The child is definitely aligned with the Authority, you can see one being destroyed in the subsequent chapter prior to a massive exterminator attack. I always imagined it was a similar model of Safeguard as Sanakan as they both have incredible regenerative ability and deadly in close combat. I imagine that the child Authority could assume an adult battle form as well, given the resources and reason to do so.

On that subject...

I've taken the forehead Safeguard symbol to imply a direct connection with the Safeguard commanding body. We rarely see Safeguard without the symbol but here are the occasions:

- Sanakan in child form (can enter Toha which was restricted to Safeguard at that time), the symbol appears as she enters battle form
- Cibo after she's hijacked Sanakan's body (the symbol returns as Sanakan regains control)
- Dhomochevsky and Iko do not have the symbol despite being Safeguards. Also the exterminators Dhomo summons do not have the symbol. It is well known that they were cut off from the rest of the Safeguard.
- Sanakan no longer wears the symbol after she resumes her battle form in the last volume and is also an independent agent at that time.

It tends to imply that the symbol is strictly an indicator of the entity's current connection to the Safeguard rather than the origin of their body or previous affiliation with it. Sort of like a real-time brand or badge. I can't think of a single exception to this during the entire manga.

This has led me to believing that the Authority are 'lost' Safeguard that have been cut off from the commanding body of the Safeguard for whatever reason and are more or less 'feral' and simply running amok. This would explain their odd behavior on that level and seeming lack of objective. The Safeguard are probably completely unaware of them.

Lastly, I always had this theory that the reason the kid didn't attack Killy and in fact gave him food was that he recognized him as a fellow Safeguard. The Authority never directly attacks Killy.

But Sanakan was useless in child mode. And I think Safeguard technology became more advanced the higher up Killy went, so I can´t imagine the child being such a poweful safeguard so deep down. And lastly Safeguards need conversion towers nearby to regenerate or even spawn the low level ones.