An excerpt from the novel, Killing Star.

The great silence (i.e. absence of SETI signals from alien civilizations) is perhaps the strongest indicator of all that high relativistic velocities are attainable and that everybody out there knows it.

The sobering truth is that relativistic civilizations are a potential nightmare to anyone living within range of them. The problem is that objects traveling at an appreciable fraction of light speed are never where you see them when you see them (i.e., light-speed lag). Relativistic rockets, if their owners turn out to be less than benevolent, are both totally unstoppable and totally destructive. A starship weighing in at 1,500 tons (approximately the weight of a fully fueled space shuttle sitting on the launchpad) impacting an earthlike planet at “only” 30 percent of lightspeed will release 1.5 million megatons of energy — an explosive force equivalent to 150 times today’s global nuclear arsenal…

The game plan is, in its simplest terms, the relativistic inverse to the golden rule: “Do unto the other fellow as he would do unto you and do it first.”

Presumably there is some sort of inhibition against killing another member of our own species, because we have to work to overcome it.

But the rules do not apply to other species. Both humans and wolves lack inhibitions against killing chickens.

It’s an entirely new situation, emerging from the physical possibilities that will face any species that can overcome the natural interstellar quarantine of its solar system. The choices seem unforgiving, and the mind struggles to imagine circumstances under which an interstellar species might make contact without triggering the realization that it can’t afford to be proven wrong in its fears.

They won’t come to get our resources or our knowledge or our women or even because they’re just mean and want power over us. They’ll come to destroy us to insure their survival, even if we’re no apparent threat, because species death is just too much to risk, however remote the risk…

The most humbling feature of the relativistic bomb is that even if you happen to see it coming, its exact motion and position can never be determined; and given a technology even a hundred orders of magnitude above our own, you cannot hope to intercept one of these weapons. It often happens, in these discussions, that an expression from the old west arises: “God made some men bigger and stronger than others, but Mr. Colt made all men equal.” Variations on Mr. Colt’s weapon are still popular today, even in a society that possesses hydrogen bombs. Similarly, no matter how advanced civilizations grow, the relativistic bomb is not likely to go away…

We ask that you try just one more thought experiment. Imagine yourself taking a stroll through Manhattan, somewhere north of 68th street, deep inside Central Park, late at night. It would be nice to meet someone friendly, but you know that the park is dangerous at night. That’s when the monsters come out. There’s always a strong undercurrent of drug dealings, muggings, and occasional homicides.

It is not easy to distinguish the good guys from the bad guys. They dress alike, and the weapons are concealed. The only difference is intent, and you can’t read minds.

Stay in the dark long enough and you may hear an occasional distance shriek or blunder across a body.

How do you survive the night? The last thing you want to do is shout, “I’m here!” The next to last thing you want to do is reply to someone who shouts, “I’m a friend!”

What you would like to do is find a policeman, or get out of the park. But you don’t want to make noise or move towards a light where you might be spotted, and it is difficult to find either a policeman or your way out without making yourself known. Your safest option is to hunker down and wait for daylight, then safely walk out.

There are, of course, a few obvious differences between Central Park and the universe.

There is no policeman.

There is no way out.

And the night never ends.

All thanks to this spectacular Quora answer.