I used to get very emotional when I saw cats (whether my own,or someone elses) with thier catch of the day.One day I was discussing this with someone,after seeing my neighbors normally docile kitty brutally kill a young rabbit in the field across the street from where I was living.He brought up several points I had never thought about.
1.) Most small prey animals(rabbits,rats,mice)breed year round,and if not for the great number of predatory animals that kill them,we would be up to our armpits in them.
2.) The cats are actually doing Mother Nature a favor.If the babies are so poorly hidden(they should be in underground warrens when momma isn't around),that kitty could get them,then the mother didn't have the instincts to keep them hidden,and would have passed this on to the babies.Kitty just removed a poor genetic strain from the gene pool.
3.) If momma was around,she didn't have the instinct to fight for her babies.Once again,this is a trait that would have been passed on.And yes,a wild rabbit should fight for its young,or at least distract the predator,giving the babies a chance to get away.
4.) If the babies were near the warren,but didn't bolt for it as soon as they knew there was danger(they should have heard,or smelled the cat),then,once again,they didn't have good survival instincts.
It is a case of survival of the fittest.But,we humans tend to think that because that rule no longer applies to us,it shouldn't apply to anything we think is cute,either.But we've managed to kill off most of the animals who would have made a meal of bunny.In the US,all the feral cats,and outdoor cats put together will never even put a dent in the rabbit population.
The biggest worry is disease.Most prey animals carry something,and whatever preys on it,is likely to catch it,or at least become exposed,and pass it along.And many of these things can be passed to humans.Hanta-virus(out west), leptosporosis(in rabbits),rabies,tapeworm and disease carrying fleas,Lyme disease and Rocky Mountain Spotted Fever carrying ticks, on down to roundworms,ringworm, and any number of other things can be picked up by a cat or dog,from a prey animal, and brought home.