I recall reading somewhere (Ringworld, perhaps?) that the explanation to the Fermi Paradox is that anyone who sticks their head up over the wall gets it shot off.
Fact is, the Universe would be a terribly lonely place if we were all alone. I suppose we're willing to risk destruction/enslavement by a superior or hostile alien civilization just to not have to exist all alone.
The "beserker equilibrium" hypothesis is a fun thought experiment, but it seems unlikely to me. Robin Hanson did a particularly good job writing about this here:
My hypothesis is that we are alone and the creation of sapient life is a very very rare thing.
The chance of life appearing, on top of the chance of the life being capable of getting more complex (multi-cellular), on top of the chance of life getting sapient, without counting the chances of the uncountable iterations between these steps...
There are too many stars, too many galaxies, too many galaxy clusters, and we've been here too short a time for me to believe that we're the only life in the universe.
Even on this one planet there are more ways to be alive than one can imagine, and those ways, once embodied, are very, very persistent.
Even forgoing the other comments on the number of universes/galaxies/stars/planets in existence, how do you reach this hypothesis given that you have only one example of evolution to draw inferences from. The entire train of thought is derailed that the only reliable information we have on evolutionary systems is our own system and, using this information, we can only conclude that every planet on which we have found microbial life we have also found sapient life.
Divide the known history of the Earth into ten million year increments, and the fraction of them with life is quite high compared to the fraction with intelligence.
It's a compelling idea. It takes someone with enough technology to accelerate an asteroid to a reasonably percentage of C, and aim it accurately enough. If a single expansionist civilization that is xenophobic enough reaches that technology level, anyone else could expect to suddenly find an incoming planetkiller at a speed that'd make it really hard to do anything about it other than try to get as many as possible off-planet. If the victims manage to save enough people to remain viable, presumably they'd be rather shy next time out.
Fact is, the Universe would be a terribly lonely place if we were all alone. I suppose we're willing to risk destruction/enslavement by a superior or hostile alien civilization just to not have to exist all alone.