All shelter workers can tell horror stories about cats that come in tarred and feathered, burned, or tortured in some other way by cruel kids or disturbed adults. A cat outside is a likely target for people who collect animals to sell to research laboratories. Outside pets are at the mercy of the people they encounter. The HSUS speculates that over 100,000 cats are caught in traps each year. Those who aren't killed may suffer for days before being released and often lose limbs from the injuries.
Most outdoor cats die from auto accidents.
Cats are intelligent and alert, but they stand very little chance against fast-moving vehicles. Anyone who's ever worked in a shelter knows that unaltered cats allowed to roam and mate at will account for millions of the cats euthanized each year. One female cat and her offspring can produce 420,000 cats in seven years. All pets, whether strictly indoor or indoor-outdoor, should be spayed or neutered. Pet owners who allow unaltered animals outside are irresponsible and at the root of the terrible pet overpopulation problem resulting in millions of animal deaths yearly.