My interpretation of the ending...
Now I know that the most obvious factor would be to assume that Sarah stabbed Juno in the leg, out of vengeance for technically killing her friend (who's name I can't remember). To be fair, Juno did stab that girl on accident. It wasn't intentional. In a situation like that, what good would it have done for Juno to admit to what she did? At that point, I think it would be really irrelevant. So I don't entirely view what happened as Juno's fault. Overall, she was in a completely insane and nightmarish situation. But still, it was pretty messed up.
I think Sarah left Juno to die, because during the end, she had a wicked moment of clarity, understanding that the situation that she was stuck in, was really all Juno's fault. Maybe Juno wasn't entirely responsible for stabbing that girl in the throat...but she was certainly responsible for Sarah being in that situation...due to her overwhelming need to explore an undiscovered cave system, which ultimately came with an extreme set of risks and consequences. So really, the whole situation was Juno's fault, and in that moment, Sarah recognized that. And more importantly, she trusted Juno, and that trust became shattered. After all Sarah had been through, losing her daughter & husband, I think being trapped in a cave full of a bunch of demonic reptilian creatures was the last straw on her sanity. She mentally snapped, and took her aggression out on Juno for being the only one who created the situation that she was in. Like I said...it was an intense feeling of 'broken trust'.
And on top of that, I think in that final moment, Sarah came to terms with the fact that Juno was once having an affair with her husband. I think Sarah knew that the entire time, but due to the severe trauma she experienced, she sort of went into denial about it. And yet again, for Sarah, regarding Juno, that was just an extra smack in the face.
Was Juno innocent or guilty? You decide. Thoughts? Feedback?