>>64509806
>They should have been punished for it severely.
They were on several occasions, with the Tyranids, the Dark Eldar, the Orks.
But here's the thing. It costs you a handful of diplomats to try to make peace. If it doesn't work, you lose a dozen people. If it does work? You save the lives of millions and get billions more without a shot fired.
Attempting diplomacy is the pragmatic, cold, logical choice. That the Imperium refuses to do it shows that they are less concerned with survival than they are with their ideology.