Oh, so you're one of those people that thinks the AI should always and only play the most optimized possible way and that the game's story aspects are useless filler?
Gotcha.
Yep. That's me. Well known around this community for hating stories and never role-playing. I never even wrote gameplay guides for Paradox franchises that involved elements of narrative. They aren't even available for sale even as we speak.
Leaving aside the false dichotomy and hyperbole for a second, let's consider for a moment that maybe the AI could do both. Maybe it can play in some kind of optimized way and roleplay a bit (in the limited sense that programs can do that sort of thing).
What if, and I'm just spit balling here, you could program some new AI personalities that might be willing to
beg for subject status when faced with an empire that purges on a regular basis
and they are much weaker. Not form alliances with them. Not form federations with them. Not sign non-aggression pacts. Not trade. Just be willing to offer themselves as a subject when an empire with a history of purging is nearby and much, much stronger than them.
That behavior might make some sense and have role-playing possibilities.
I would also accept a diplomatic "reset" if the regime of genocidal lunatics is replaced by something new. That also makes sense.
Here's what doesn't make sense, and what would make for really stupid gameplay:
"Well, the Bugs have murdered untold trillions. They are offering us a non-aggression pact."
"Hmmm. Aren't they threatening several empires in the western part of the galaxy?"
"Yes."
"Haven't they basically committed genocide on all populations they conquer?"
"Yes."
"Do we have enough military might to ally with other empires to try and oppose them?"
"Sure."
"Well, these wars can't possibly be a prelude to the acquisition of more planets, resources, and fleet capacity to eventually wipe us out. Sure. Accept their offer."
Gah, I'm so tired of seeing this type of argument. Let's just jump to the natural conclusion -> yes the AI should wreck you as soon as it has more ships, it's the safest and most sensible way for them to play.
If I have a track record of genocidal behavior? Of course it should.
Also, in Stellaris, conquest and colonization is very profitable. If the AI is not going to at least
try to take advantage of this, it's going to get its clock cleaned all the time.
AI personalities with any kind of desire to expand should do just as you described.
Compare this to a FE awakening- I had this in my latest game. A Xenophile FE awakens in my galaxy. Instead of; insulting it, declaring it a rival, severing all potential ties, I maintain cordiality. I know that- should nothing change in the time it takes them to get to me- I'm at best never going to be independent again. However, things change. I expanded, maintained a large fleet and rushed unbidden techs. The unbidden did in fact invade, and by the end of the game we were in a federation together.
It's a xenophile AE. Making nice with it is sensible.
Hell, at least the xenophobic militarists, when they wake up, offer you the option of being a thrall.
Contrast this with a normal-powered empire that exterminates people. Why make nice with it, unless you are so much weaker that resistance is futile?
You reference realpolitik. What you're suggesting is the exact opposite, opposing purifiers because of morals. There are plenty of reasons to bide your time and make sure you're not next on the plate.
Morals?
There can be only one end game with such a polity: a future war of extermination.
Even if you choose not to exterminate them (no need, really), it will be coming to you.
So, I will reiterate. Unless the empire in question is so much weaker that it can only survive by being a subject, it should not be making nice with genocidal empires.
Declaring an offensive war against such a power is a different question, but signing non-aggression pacts? That's insane. In Stellaris,
you can't attack someone even after breaking the pact right away. You sign a non-aggression pact with them, it's 10+ years until you can attack them. They lose an important battle in a war that cuts their military strength in half? Too bad, you signed that pact.
Trading with them? That's a terrible idea, too.