There would be a reason to destroy Earth if you were an alien species and the reason some scientists say we should silence our communications to the rest of the universe. Namely, to eliminate us as a potential threat/competitor. SETI has written about it themselves. The example given is from Greg Bear’s novel, The Forge of God:

In Bear’s story, the Earth is visited not by laser-wielding aliens but fully autonomous Von Neuman machines. These are self-replicating interstellar probes that spread out among star systems using the resources they find to generate more versions of themselves. The premise of Bear’s fictional Universe is that with the laws of physics keeping interstellar travel slow (it takes a long time to travel among the stars even at near light speed) all intelligent technological species will, essentially, be in the dark about what is out there.

Thus for some cultures the best defense will be a good offense.

The Von Neumann machines of Bear’s story are killer probes. Once our stray radio broadcasts are picked up, the probes arrive, try to build more versions of themselves and, essentially, disassemble our planet. No explanations. No monologue of evil intent. Just mindlessly eliminating potential competition and potential threats. Then it’s on to the next system.

http://www.setileague.org/editor/wolves.htm