It depends on what sort of people the cat owners are. If they were the sort who'd likely retaliate by slashing the car tyres or lobbing bricks through my window and generally make life hell, I most certainly wouldn't say a thing

. If they were reasonable, level headed understanding people that would be different. Cats wandering loose are likely to get run over, attacked, caught/killed if it climbs into a garden where there's a dog, have accidents, owners take that risk when letting them out so can't really come wailing if something happens as they put them in that position. I doubt you'd get in trouble if a dog kills a cat unless you deliberately throw it to the dog, anymore than you would if it killed a rabbit , you wouldn't know until it was too late. Yes it's unfortunate but that's nature, it isn't your fault.