None of those races were corrupted in their entirety for one, and none of them willingly apart from rouge individuals (Saren/ TIM). All the geth willingly joined the Reapers as allies in the extermination of all advanced organic life in the galaxy, except for perhaps Legion (and we aren't even sure of that considering its runtimes carry Reaper code upgrades like all others).
Self preservation isn't a valid excuse until it is shown why all other possible options (the most obvious one being a retreat from all contested quarian systems such as Rannoch) were not taken.
The geth are not being held to a different standard. If the Reapers showed up at any other race's planet and their government were to ally with and serve the goals of the Reapers without being indoctrinated for any purpose, anyone who fought for said government would be an enemy in need of destruction, no questions.
Where is it said that they agreed to kill all organic races along with the Reapers? From what I can see, it's not explicitly stated exactly what the Reapers' offer was. But the fact that the Reapers uploaded a virus to the geth consensus suggests that there was never an intent on the geth's part to remain a willing ally. The Reapers intended to destroy both the geth and the quarians. Once they discovered the Reaper virus, they appealed to Shepard for help in releasing them, which was a move that helped both the geth and the quarians. If their intent was purely to support the Reapers, then they would ignored any attempts to communicate from Shepard and promptly proceeded with wiping out the quarians. They certainly wouldn't have raised their shields and deactivated their guns.
As for why they didn't evacuate Rannoch, there's not really enough evidence to make a clear distinction if that was intended to be some kind of declaration of war. The quarians never attempted to communicate with the geth about it, and any reports they did receive about the quarian's status suggested they still had every intention to destroy every geth in the galaxy. Why give a potential enemy power if they intend to use it to destroy your own kind? At the very least, they could have still been trying to reach a consensus or holding out for the chance that the quarians would negotiate. The quarians launched a surprise attack on them without any prior communication, so we don't know what they would have done if the quarians attempted negotiation. Tali herself was the only one who tried, and that hit a blank wall as soon as the Admiralty had a hand in it.
They couldn't just go anywhere. The Council still had laws against AIs. There would be a much larger war against them had they left "geth" space. Same reason why the quarians couldn't just let them go free when they first showed signs of sentience. The quarians created the geth in their territory, so by Council standards, they probably see the geth as having a right to remain there regardless if peace between the two races has been established. Negotiation was the best option aside from outright genocide. In their position, Rannoch/quarian space was the best place to achieve both.
I think we still have different opinions on "corruption" too. I saw TIM as a tyrant, but his ideas to use Reaper tech wasn't unwarranted. Reaper tech is just that--technology. It's the AI (Catalyst) and their platform that determines how it's used. Additionally, it's not tech that the geth are using, it's coding. No external upgrades, no risk of indoctrinating organics. We've already seen in the consensus that the geth are capable of identifying Reaper viruses, and they can use that to their advantage. If the Reaper code functions by allowing the geth individual processes for each platform, there's no concerns of corrupt one = corrupt all. Post-upgrade, their platform is probably more secure than EDI.
But really, if the main reason to destroy the geth is the fear of advanced technology (like EDI, the mass relays, the Citadel), then that's perfectly fine. The universe will function either way. I would see it as an opportunity to improve the way of living though. My optimal outcome is what occurs in-game after establishing peace: the quarians learn from the geth (though I don't think it would occur as easily as depicted, they should be appealing to the Council and working out the kinks), and their technology helps improve their ability to adapt. Species will go at it either way in the future, might as well reap the benefits while you have it.