Question: What sort of woods are we in before we light? Related.[0]
[0] Beyond “Fermi’s Paradox” XVI: What Is the “Dark Forest” Hypothesis - https://news.ycombinator.com/item?id=27841361
If everybody in the forest is afraid of strangers, where are they? Hiding in the dark, right?
In that case, without additional assumptions, someone who sees a fire can't just attack; they have to consider the fire starter might have friends in the dark.
Assuming it would be relatively easy to sterilize Earth with some sort of kinetic weaponry, there's no way to ensure that most humans live on Earth, from afar.
For all these hypothetically aggressive aliens know, 99% of humanity could be in the Oort cloud already.
So blowing up the potentially life bearing planets would be exactly as dumb as hitting a massive hornets' nest with a shovel because they might be (are!) mean.
Now you might say "but nobody wants to live in deep space!" - well, we don't, but interstellar travel requires that we learn! And once you are thoroughly adapted to living without a planet or sun, why go near them? They're unstable and dangerous. It's illogical to assume that any interstellar spacecraft will ever reach its destination, exactly because to be successful it has to be sustainable over the course of the journey and the occupants will change their minds.
Hypothetical aliens can reason this out, and can assume that we can reason this out, and therefore assume that in general, life that huddles close to a star must progress to a different lifestyle if it's going to get anywhere. So it can't ever be safe to smack a planet when you can't know if all of the inhabitants are still in the nest.