But ME3 was a great place to start.
Also the reductio ad Hitlerum can also be used like this.... (note this is an example and not directed at anyone) Hitler liked classical music. So because you think that schools should teach about classical music you're a Nazi. 
Yeah, that's similar to accusing someone to being a nazi for liking a specific flavor of icecream. Sure, it can be used for that, it's also godwins law useful.
However when we're discussing genocides, experimentation on prisoners and other such things, then it's not applicable. I can't see anyone discussing music and the schoolsystem and comparing people to nazis due to musical preferences. Nor is it being used in a similar way.
The w
arcrimes in the Morningwar and ME3 would make Hitler and his followers look like shoplifters in comparison. Billions VS millions. Then again, I think a lot more people would have died if Hitler hadn't been stopped.
It's also interesting how Tali says Billions in the elevator and the codex says millions. I don't think the writers put that much effort into thinking these thigns through. They just threw something out there then let it float.
People had a conflict, they tried to kill each other, people died, one side eventualy retreated, that's it.
I don't think they expected people to actualy wonder about the details that much.
Even Talis and Legions accounts are very much "up in the air" with a distinct lacking of detail. You don't get exact dates, times, numbers. What we get is, fighting, deaths, reatreating, hate.
"We talked" "Then the Quarians attacked" "Then we stopped talking" "Then that happend" "some time long, long ago something happend".
@Deinon
From what I've seen the Quarians were the agressors that started the Morning war. They were running tests and killing Geth(with plans to kill off every last Geth), eventualy after the shooting had been going on for a while the Geth started fighting back. The Geth are never said to have experimented on Quarian prisoners.
The Quarians resumed experimenting on Geth somewhere along the events of ME2 if it wasn't going on before that point.
It's likely that the Data that Tali recovered from that Geth that helped you expose Saren, spark Raels interest in aquiering more Geth data. That data that Shepard could give to Tali to pass on for the pilgrimage. I think she still has it and hand it over even if Shepard doesn't give the permission. She also gives the Quarians technical specifications on the SR-1 stealth systems which enables the Quarians to send in frigate sized vessels for Recon and recovery. (If they wern't doing it before that point)
In between ME1 and 2 (Mass Effect: Ascention) the Quarians order explorer flets to be formed to find either a planet or Reaperships or tech that could help help them take back their homeworld.
As for their percived necessity to retake
Rannoch:
Also, the physical needs of the Quarians doesn't include pollination by plantlife. The plantlife however requiers them because there are no insects(bees) to spread their pollen. They can adapt to a new Dextro environment, even if it takes more time it won't kill them as a species. It wouldn't be more than an inconvenience. It's not a physiological necessity to retake
Rannoch, but rather a convenience.
Their cultural "necessity" is a luxury, and not a necessity.
-The Quarians started the Morning war.
-The Quarians started the war in ME3.
-The Quarians Experimented on Geth in the Morning war and in ME2-3.
-The Quarians killed billions of Geth in the morning war and the war in ME3.
-The Geth responded by defending themselves and securing their environment from hostile lifeforms bent on killing them.
-The Council attacked and killed unarmed synthetics on the Citadel that had requested an audience during the morning war.(their origin is irrelevant, they were synthetic lifeforms, that made the council an untrustworthy negotiator since they lacked neutrality)
-Every organic race sent every fleet and everything they could spare to the blockade of the Perseus Veil. Displaying that they were a military threat to the Geth that could, potentialy attack at any moment.
What we lack, is exact dates, exact numbers and better answers. The codex says millions of Quarians died fighting the Geth, what killed the rest? Their physiological weakness? Starvation? Civilwar? Diseases? A combination of all of that? What were the Geth cleaning up after the war? What was it that took three centuries to clean up, and they still wern't done cleaning it up?
How long would chemical/gas weapons last before the environment breaks it down? Biological? Nuclear?
Comparing it to what the Krogans did to their homeworld doesn't mean much, it's hard to beat the Krogans when it commes to destroying your own homeworld. You would have to be Drell for that.
The Drell managed to kill Billions of their own people and all life on their planet, including the plankton and every microorganism. (which simply seems impossible, but that's the description given)
Saying that the Geth killed 99% of the Quarians in one on one combat seems crazy to say the least. The Unified Quarian Government had ordered the destruction of the Geth on Every Quarian world, every Geth in existance.
How many Quarians there were that had a problem with it, we don't know.
They were treated as traitors and enemies of the state however where the military would use lethal force and even explosives and heavy weapons to achive their goal of destroying all Geth.
Apparently different Geth joined the Geth rebellion at different points in time, that Geth who's owner didn't want to hand over his Geth, pointed out that it hadn't jouned the Geth resistance. He was told it doesn't matter to them.
The Geth didn't act as one initialy, but as they got put under pressure they eventualy realised there was no other choice. Because the Quarians wouldn't spare anyone.
But, really, the morningwar is as much a writer slouth as the ME3 writing. Also, Quarians can't be compared to humans or Krogans when it commes to surviving hardships. If the situation is anything but optimal they will start dying like flies. Therefor their situation can't be compared to WW2, The Great Famine, or The Black death. Their deathtolls would have been massively worse from any of those situations.
Even if the Geth only killed millions of Quarians in selfdefence it still sounds bad, but it's not like they had a choice other than accepting their death sentence. Selfdefence gives you a lot of leverage when it commes to what is right and wrong.