We definitely need more information about what the Hishmeri are doing in that region of space- which is exactly the mission the
Atuin was involved in. The only real catch is that one explorer can't possibly map out an entire region as fast as a whole fleet of Hishmeri scouts can... which is why the Hishmeri beat us to the punch.
or to avoid leaving an enemy behind them they finish up by exterminating the natives from orbit. We don't know there MO and given it's already really pointless to raid a prewarp civilization I would not be surprised if it was pointlessly cruel.
That doesn't make any sense.
Firstly, by definition a prewarp civilization can't chase your interstellar nomads, and by the time they
invent FTL travel you'll be long gone and out of their reach. No one like the Hishmeri should ever be worried about "leaving an enemy behind them" that way. You're trying to cram an Evil Idiot Ball down the Hishmeri's throats here, and while that may fit some preconceived notion of how they "should" behave, it's not necessarily how they
do behave.
Secondly... There are a host of reasons why a Hishmeri Sept might
VISIT- not raid, visit- a prewarp planet. Maybe they detected dilithium or some other valuable material on the planet. Maybe they want fresh food. Maybe they just want to get out and stretch their legs on a planet with pretty flowers and clouds and things you can't get while flying around the galaxy in a big tin can. Maybe the Hishmeri are a multi-racial collective and they routinely stop by people's planets and say "hey, do you want to come with us on our space road trip?"
All we
really know is that the Hishmeri have gone to the planet. We do not know what they're doing on the planet, or what their motives are. Their motives could be horrible, or relatively mundane and harmless. The actions they take to fulfill those motives could
likewise be horrible, or mundane and harmless. We know that the Hishmeri Septs
sometimes use violence to get what they want, but that doesn't mean they
always use violence. In a case like this, they might even threaten violence to get something relatively minor (like a hundred thousand tons of shiny rocks), and just
move on, and the pre-warp civilization might well comply with the threat because, well, giant orbiting battleships are pointing mountain-incinerating beam cannons at them.
If we come back a quarter later and it turns out the Hishmeri have taken a hundred thousand tons of shiny rock from the locals at disruptor-point then moved on, but otherwise left the locals basically unharmed... do we need all out war to the knife against the Hishmeri species because of this?