I didn't read it as competition for scarce resources. I read it as preemptive war against an existential threat. The Dark Forest suggests that any alien species

I didn't read it as competition for scarce resources. I read it as preemptive war against an existential threat. The Dark Forest suggests that any alien species presents a potential existential threat, and the safest thing to do is to (if possible) exterminate them before they even know you're there. It's kind of related to the Prisoner's Dilemma. And you don't need every species to play Dark Forest, either. As long as one species is playing Dark Forest, all the others learn to hide, or play themselves, or get exterminated. That's the fate of those who want to be nice.