Hitler was the aggressor all the way through. That's quite a bit different to a species willing to engage in mindless murder against every last man, woman, and child without any efforts to communicate any kind of intent.
by what objective metric are you defining "agression"? He subjectively declared a certain subset of people to be more worthy of moral consideration and deemed all actions contravening the maximum utility calculation for the preferred group by the Untermensch (such as a Jew having the audacity to be a banker or scientist controlling Aryan money/ technology) to be aggressive action against it.
That's not trolling, that's identifying coherent differences in circumstance. Collective responsibility is being assigned to a collective entity. There are no non-Rachni (that we know of) hiding out in civilian cities fearing for their children. Identify a clear-cut civilian Rachni population who was impacted and I'll concede the point. .
It would seem that a basic understanding of life-cycle sciences and demographics would render your assigning of collective responsibility and value judgement upon moral worth to every individual of an entire species to be quite ridiculous here. This might apply in the case of something hiveminded like the Borg, Necromorphs, Geth, Tyranids or Skynet where there is little distinction between the two and "individuals" are just organs of a collective with little to no actual agency, but there definitively is for the rachni even if you include all the drones under single queens to be a single individual. The single natural one we meet certainly isn't mindlessly genocidal
I did describe ones "that we know of", unborn, physically immature children, queens without access to any drones etc. are by definition incapable of resistance and comparable to "civilians". Killing them serves no legitimate self defense purpose. This before the fact that we aren't really given much insight into rachni society, but any person with even a small amount of sense would probably conclude that a species capable of such technological wonders as building spaceships that circumvent general relativity (something even modern humans are incapable of) and creating music probably isn't a bunch of mindless bugs living in caves with otherwise no civilization. It took humans 50,000 years of such to develop the technological, social and culture infrastructure to get to where they are today. You don't just go from "mindless" bug to magically evolving the ability to understand highly advanced technology like FTL drives or the Crucible (which they contribute to in ME3).
Hitler scenario: a maniac willing to murder an entire group of people who have demonstrated no sense of aggression for his own deluded sense of power.
Rachni scenario: genocide against a genocidal enemy willing to execute your entire population, with no indication of stopping under any circumstances.
As explained, your operative definition of "aggression" is subjective and therefore a meaningless distinction. Mine would include the ability to inflict harm, not just the will to do so. I can't just kill a toddler that desires aggression against me and hits my leg because I denied it sweets. The punishment does not fit the crime. Similarly, hunting down an executing individuals who are incapable of resistance unless you deliberately invade their planet and hunt them down to every corner of it is not morally justifiable, and slaughtering their noncombatants well past the point of military necessity definitely isn't.
An infintisemal number of individuals could then monitor Suen and report back any signs of reacquiring FTL capability, wherin the Council could deal with this in a manner that didn't involve Starship Troopers esque "kill em all". Hell, the Council devotes this listening post outside the Rachni relay even after genociding them from existence anyway.
Rachni scenario: genocide against a genocidal enemy willing to execute your entire population, with no indication of stopping under any circumstances.
The indication of their stopping to any sane individual would have been when you are in a fleet of spaceships and they are incapable of reaching you on a toxic planet you have literally no need, nor use for. Again, humans aren't going into the forests to deliberately exterminate wild animals that would attack them on site if they could.
Edit: And claiming the Codex as unreliable in lieu of another source is an invalid argument. This is further supported by Leviathan/the Reapers themselves playing a role in driving the Rachni collectively insane.
You just explained the other source that renders codex description inadequate. The fact that the Council were essentially hunting to extinction an entire species of sentients brainwashed and unethically modified into insanity against their will into engaging in self destructive violence, who are therefore not even responsible for their actions at all (as you were claiming earlier with the "collective responsibility" garbage) makes it all the more repugnant. I'll concede, however, they certainly had no knowledge Reapers or Leviathan this at the time, so it doesn't make the Council's actions any more reprehensible than simply killing an entire sentient and sapient species down to the last individual out of ideologically motivated racial violence.