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?