[The Guardian - opinion]

Cats kill billions of small animals a year, putting animal lovers in a
fix -- how do you reconcile keeping a predator as a pet?
...
There have been two serial killers in my life. The first was a former
student. A couple of years after he graduated from my university, he
murdered his father, his mother, his younger brother, and the family
dog. After he was arrested, the local television station sent a
reporter to interview me because I had been his academic advisor. When
the reporter asked me what he was like, I stupidly looked at the
camera and mumbled the classic cliché:
...
The same could be said of the other killer in my life, my cat Tilley.
She spends part her days outdoors, and like most cats, she is a
recreational hunter. I am usually successful in suppressing the guilt
that comes with having a serial killer for a companion animal, but a
recent report in the journal Nature Communications has caused me to
rethink the ethics of keeping predators as pets.

Based on existing data, the researchers concluded that the havoc
wreaked by cats on native animal populations has been vastly
underestimated. They calculated that in the US, cats kill between 8bn
and 24bn small, feathered, and furry creatures a year, and are the
largest human-related source of mortality among birds and mammals.
While most of this carnage is caused by free-ranging stray cats, it is
nearly certain that pet cats are responsible for at least 1-2bn of
these deaths.

Are tabbies in the UK as deadly as their American cousins? Probably. A
2003 study of cats living in 600 British households found that over a
five-month period, the cats brought home the carcasses of over 14,000
small animals. With 10m cats living in British homes, the numbers add
up.
...
Putting a bounty of feral cats would, of course, be unacceptable to
the millions of us who are cat lovers. An alternative cat reduction
strategy has emerged in recent years – "trap-neuter-return" programs,
in which free-ranging cats are captured, neutered, and set free. Often
these animals live in groups under loose human supervision. As you
might expect, bird enthusiasts are not happy with the proliferation of
these "cat colonies", and indeed, a recent survey found that cat
colony caretakers and bird conservation professionals live in
different moral worlds. For example, while 90% of birders agreed that
feral cats contribute to the decline of native birds, only 20% of
cat-advocates agreed.

The birders, it seems, are right. "Trap-neuter-return" programs may
eventually reduce the numbers of free-ranging cats, but they will
probably take decades to have an appreciable impact. In the meantime,
billions of wild birds and mammals will die and some species will
become extinct.

Jesse was faced with the prospects of either getting rid of her
beloved pets or living in violation of her convictions. The existence
of millions of feral killing machines in our alleyways and backyards
poses an equally unpalatable dilemma.

--
full story:
http://www.guardian.co.uk/commentisfree/2013/feb/01/ethics-keeping-killer-cat