Let's assume there is some advanced civilization that doesn't want competition and kills any aliens it finds. Would it make more sense for them to wait for an alien race to develop technology and be on the edge of being a threat before they take action, or would they just destroy the biosphere of any planet they find that could eventually evolve intelligence? My bet is they wouldn't wait around for a threat to appear, they would prevent it from forming in the first place. With that in mind, we are on the verge of being able to detect if nearby exoplanets could support life. A much more advanced race should be able to do this for every star in a wide area around it, if not the entire galaxy.

Earth has had life for about 3.7 BILLION years. It should have been detectable for most of that. The Milky Way is about 100,000 light years across which means an advanced civilization anywhere in our galaxy should have known that earth supports life for basically its entire existence.

So we are left with three possibilities: Advanced aliens exist and don't care Advanced aliens exist but are waiting for us to be a threat before doing anything Or advanced aliens don't exist.

I find option two to be very unlikely.

This isn't the only problem with dark forest theory though. If life is common enough that intelligence pops up everywhere and competition for resources ensues then no matter what kind of FTL travel you may or may not develop it is inevitable that you will eventually run into a civilization more advanced than you are. Unless you can instantly occupy the entire universe at once, someone somewhere will have been far enough away to develop more than you by the time you met then. In such a case you want to be seen as a nice neighbor, and not a locust, or you risk extermination.

As to your point of not being able to look for life across the galaxy from a single solar system, I think you vastly underestimate the power of technology. We already have plans for how to survey every star system in the galaxy. The telescopes required are just several orders of magnitude bigger than our current best. Making them would be an economics problem, not a physics or engineering problem. But if a race is at the point where they can destroy a biosphere from light years away they are necessarily at the point they can build those telescopes.