OK, I'm going to be the fan theorist and just make a guess, you devs don't need to answer if you want to leave it to the imagination. I'm guessing the monster is either depression or addiction, and the cave is our mind in our darkest hour.
At first I thought the monster was us, as if our subconscious, or how we may perceive ourselves. But then it drops the line about me loving it even though I have not learned to love myself. And there's a suggestion of self-harm so both depression and addiction tracks. But then again, the "hate" ending left me wondering...
Anyway. If any other player feels like it, share your own interpretations down here, I'd like to know what everybody else got from this.