Eh maybe I'm asking for too much but I very much dislike the black and white relations system as it is now.
You are. I've seen a lot of threads like this basically since Stellaris launched. People want to have their cake and eat it.
If you go on genocidal rampages to annihilate an entire species then
everyone in the galaxy should rightly be feeling
very threatened by you
. Even if they do think aliens are inferior to them.
I'm betting they approve that there are less xenos, but are worried that some empire has the power to go through with it. Plus, they're not purifiers. Xenos are very much beneath them...but still not necessarily worthy of being systematically killed.
It is worth noting that the fanatic purifiers do not care less about you having genocided anyone, neither lowering or improving their opinion of you.
Fanatic purifiers make their intentions perfectly clear from the beginning, the moment they meet anyone. If you are normal (even fanatic) xenophobe then you basically have the choice between taking that in an isolationist direction, the slaver path, or purger.
The moment you start purging species you show the galaxy that you're no different to those purifiers after all. It is not something that should ever be easy to get away with. After all, you have built yourself an enormous powerbase, your mighty fleet has just conquered a nation, it's neighbours are feeling threatened, but then you genocide the populace. The galaxy knows exactly what kind of scumbag you are now.
For the record, the opinion penalty does decrease with tube and scales to the extent of your purge, so if it really is a minor little couple of pops purge, then the galaxy will get over it in time. But yeah, when you've massacred enough innocents to drive it up into the -4000 range then you are unmistakable for what you are and any nation would be foolish to think they would be safe from you in the long run.
Also, these measures help to create challenges through federations etc to slow down the juggernaut snowball effect as you keep conquering, which makes the game stay interesting into the phase where most games have already become a boring mop up exercise.