The 'Dark Forest' theory makes for great sci-fi horror stories, but it doesn't make any sense as a real solution to the Fermi paradox.

By the time a species figures out that they're in a 'dark forest,' they'll likely have already been broadcasting their location to the entire galaxy for a long, long time.

Plus, it's entirely likely that any space faring civilization will end up having the majority of its population in big ole' space habitats instead of chilling out down a gravity well. For all you know, going all exterminatus on an alien planet might only take out a fraction of them while pissing the rest off and letting them know where you live.

Any species that has gotten that far will also know SOMETHING about how different species respond to changes in their environment, and will be aware that there might be bigger, stronger species out there that don't take kindly to random acts of xenocide. It's also very likely that, while any space faring species knows what war is, they'll also know what diplomacy is, and will know that this is PROBABLY true for the other species they encounter out there.

It's genuinely not the case that a 'strike first, ask questions later' is the best or most logical course of action for a space faring species. In fact, the opposite is true. To paraphrase the hack frauds over at Redlettermedia, the perfect organism isn't something that violently and pointlessly attacks everything around it, because that usually results in its own death.