However, it might be possible that any alien species would want to destroy us - not because they want our resources or slaves or because they're evil - but because they're scared and care about their own survival. If it's possible for them to wipe us out, then for all they know, we might have the same technology. Many humans are still willing to wipe out other members of our own species just because of minor cultural or genetic differences, is it so hard to believe that we would be willing to destroy something completely alien? Look up "The prisoner's dilemma", in situations where the game is only played once, it doesn't end well if the players are rational. Societies don't get ahead by being self-sacrificing either.
There is, however, one reason it might not be a good idea to destroy other planets - if there are at least two intelligent civilizations in the galaxy, there are probably more out there somewhere. A third civilization would be able to see what happened, and trace it back to the planet it came from, and hardly anyone could blame them for delivering justice.