I really don't see this as the existential conundrum it's been made out to be
Many animals eat their own kind. Some sharks will eat their babies (so I'm told on Discovery Channel's recent "Shark Week" series, haha), and of course spiders and other creepy crawlers do the same.
Let the spiders live out their karma and let nature take its course. No need to intervene. The insect/arachnid lifespan is pretty short anyway. Say a prayer or paritta for them so they might obtain a higher rebirth.
Of course, just because we're a part of nature doesn't make it right for us to go around eating each other. Unless it's the zombie apocalypse, then it's perfectly alright