Q: how do they know they need to hide?
A: observation
Q: What is the purpose of killing off other civilizations, when there is no gain from it?
A: wrong conclusion. Killing off others leaves more resources for the victor to harvest.
Q:I could may be see conquest.
A: conquest means bringing assets to bear on a particular target in such numbers that everyone else will notice
But you are hypothesizing just silly extermination.
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Does the lion exterminate antelope (for instance) or just stalk and kill the weak who are exposed and cannot flee? One pride will drive off and kill another in competition for territory. Just use the laws of nature that animals live and die by: as above, so below and vice versa
It is just another far fetched hypothesis, as is the one that any advanced civilization has to be a peaceful Kumbaya type.
Or, the idea that any advanced civilization would have to be Marxist.
Speculation is fun.
But comparing interstellar extermination to biological evolution is silly.
Can't say the civilization hunting and killing bad guys are not out there; they may be.
I suspect it is more likely they would be a techno-war machine gone bad, instead of any rational purpose of gain.