It is not "in retaliation". It is "in order to survive, the last and only option."
How difficult is it to understand this?
I can
easily see where the confusion arises, because the
BSG writers deliberately planted the confusion by goofing around with the Cylons' motivations, therefore destroying the logic of the story while giving themselves the widest possible latitude without having to worry about all that pesky making-sense stuff. It was a cheap cop-out on their part all along.
If a bunch of killer robots wipe out your civilization, then I think it's reasonable for people to assume that the killer robots will continue to hunt them down to the last man, woman, child, and daggit. Which in turn makes it reasonable for them to try to exterminate the killer robots without mercy or a second thought should the opportunity present itself.
Yet, incredibly stupidly, the Cylons didn't actually want to wipe out all of human civilization and changed into weepy bleeding heart liberals midway through the story. Poor dears, they were merely led astray by that nasty Cavil guy.

Oh my lord, what a dumbshit plot twist. Instead of being evil psychopaths, the Cylons became evil moronic psychopaths. For them to be simply evil is dull but at least it kinda sorta works. Turning them into morons for the convenience of the plot is hilariously poor writing.
Then there was a bunch of blather about how the human race was responsible for making the Cylons mad and they deserved to be exterminated.
1. The vast majority of humans had nothing to do with making the Cylons mad decades ago - they weren't even born yet!
2. The Cylons who were "oppressed" were the metal variety, who did not at all seem to be involved in decision making, and were being oppressed by the skinjobs, who were therefore four-star hypocrites in addition to being morons and evil psychopaths.
3. Even if humans deserved to be exterminated, it's not human nature to simply accept death meekly. Even the most evil person will fight for survival. In fact, if they're evil, they'll have no compunctions about doing what it takes to survive. So why would it be surprising for "evil humanity" to fight like maniacs for survival? That has nothing to do with morality - it's instinct.
4. Even if you accept that humans are evil, certainly the Cylons are equally evil, so why take sides at all?
5. I just put more thought into the logic of
BSG in this post than the writers bothered to put into it, so why bother fighting about bad writing?
And BTW, the fact that he's married to a Cylon does not make his decision "selfish" as someone said. How was it selfish? What benefit did it bring him, personally?
It's selfish in the selfish-gene sense (Dawkins) - that people are motivated to propagate their genes (in fact, it's the genes that are being selfish - our bodies are just vessels by which the genes express their "will" to survive and prosper).
Since Athena was willing and able to bear Helo's children, his natural instinct - driven by his genes - would be to protect her as the vessel by which his genes would be propagated. And he's willing to destroy the chance of others to propagate their own genes in order to increase the odds of his own genes surviving. This is just a reflection of the tribal mentality you see among humans all the time - they can behave in a self-sacrificing manner, but generally they're sacrificing themselves for spouses, children, or other relatives, because it's in their evolutionary interest to do so.
For Helo to be really unselfish, he would have to help the Cylons yet not be capable of breeding with them. There would be no benefit to him in doing that. But killing others to protect the mother of his children? Definitely selfish.
His wife is living proof to him that the Cylons are people, and more than that, that they're not uniformly evil creatures. She's not an exception or anything - she's just a Cylon who made a choice. Other Cylons could hypothetically make the same choice she made, so it's a little much for him to condemn the entire race to extinction.
It demonstrates that the Cylons have free will, so why shouldn't they be condemned for having the free will to exterminate humanity? If they were mindless drones, then maybe they could be let off the hook. And even if they "reform" now, why shouldn't they still be exterminated for their earlier crime? A murderer isn't let off the hook just because he's remorseful about it later, or had a bad childhood, waa waa waa. He'll still be punished. The Cylons should be treated the same way.
But once again the writers futzed the logic around, allowing the Cylons to suddenly develop free will during the course of the story for no particular reason except to artificially contrive that they shouldn't be condemned for their act of genocide.
BSG was written in a highly manipulative and illogical manner in an effort to make a pre-determined point, rather than simply being a situation which the writers explore in an honest manner, to arrive at whatever point the story takes them.
Which really is my larger point: writing this bad and contrived really isn't worth fighting over.
(Which is why the person I had problems with in the episode was Athena, not Helo. You want to talk about "traitors"? Athena was ready to go along with the genocide of her own race.
Ah, but Athena was siding with the father of her children - a big, strong guy who could be a good provider and help her propagate her genes. So she was being selfish in the same sense that Helo was.
Since the definition of a species hinges on the ability to inter-breed, Cylons and humans were not in fact separate species. Nobody is being a traitor to their species. Both Helo and Athena were acting in accordance with human nature, and their behavior was no different than those of many humans throughout history.