I saw a cat killing a mouse. Could have stopped it, but didn't
"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.