A starship is a relativistic projectile that bothers to slow down. If you're going to smash a planet that acts up, you might as well just cut to the chase. All even-remotely-physically-plausible forms of interstellar travel are also intrinsically planet-busters, including the various exotic FTL drives that hover on the edge of plausibility.
Sure, but they might not be complete dicks. They might not want to kill us unless we're a threat to them.
The more I think of it the more unlikely the kill-everyone-immediately strategy seems. Suppose you're Robinson Crusoe. You've been all alone on an island for five years. One day you're out hunting and you see another human. Do you pull out your gun and shoot him in the head before he can do the same to you? Or do you take the calculated risk that he isn't a homicidal quick-draw champion?