When let outdoors, cats kill millions of animals each year. This includes mice, rats, rabbits, birds and even frogs.
The problem is that this upsets the balance of nature. Cats that would normally go hungry from lack of prey are fed by us, so the hunting continues even when animal populations are low.
If you want to let your cat outside, try feeding them quality foods with lots of protein from meat. You can also play with them in the morning and evening to stop them killing for entertainment.