I saw a cat killing a mouse. Could have stopped it, but didn't I saw a cat chasing a mouse outside of my house and kill it. I could have chased the cat away and saved the mouse's life but I just watched and didn't do anything. Not that I was enjoying what I saw but this made me think whether what I did was a sin or not. I just think I would have interfered with nature had I stopped the cat from killing. One side there is the food chain and on the other karma. I am confused. Is it a sin to not stop a predator from catching its prey?
"Chethanaham Bikkhawe Kammam Wadami" - volition is Karma


Did you not prevent it because mice are usually an annoyance and 1 less mouse makes your life better? If so, it's bad Karma. Later you may make up an excuse like "not wanting to interfere with nature". But what matters is the intention at the time.  If you were wishing for the mouse to escape, it is good Karma. If you actually saved the mouse, it's more good Karma. If you were neutral based on Upekkha(contemplating on Karma and Vipaka), it's good Karma. If you were neutral based on indifference born of ignorance, it is bad Karma.