A small price to pay for salvation.

Nevertheless, one thing was clear: the Zen-Whoberians were exterminated (either completely or half of the population).
But was Xandar (Nova Corps) or anyone else in the galaxy aware that Thanos was behind the extermination of Zen-Whoberians? In other words, was anyone aware about Thanos' atrocities across the galaxy? 

A:

It seems unlikely that the Nova Corps would be unaware of what Thanos was doing across the galaxy seeing as they appear to have been keeping tabs on him somewhat.

Corpsman Dey: Gamora. Surgically modified and trained as a living weapon. The adopted daughter of the Mad Titan, Thanos. Recently, Thanos lent her and her sister Nebula out to Ronan, which leads us to believe that Thanos and Ronan are working together.
Guardians of the Galaxy

This makeshave avenged them.
Drax: Yes. Of course, Ronan was only a puppet. It's really Thanos I need to kill.
Guardians of the Galaxy

Gamora herself even tells Peter what Thanos did as further evidence of this. It is possible she has also told others.

Gamora: My father didn't stress diplomacy.
Peter Quill: Thanos?
Gamora: He's not my father. When Thanos took my home world, he killed my parents in front of me. He tortured me, turned me into a weapon. When he said he was going to destroy an entire planet for Ronan, I... I couldn't stand by and...
Guardians of the Galaxy

On top of that we know that the Ravagers, such as Taserface, know who Nebula is.

Nebula: That’s enough killing for today.
Ravager: She's the daughter of Thanos.
Taserface: I thought you were the biggest sadist