Not only is it unethical, but you could end up with a victim of abuse. Also, if the other cat is not neutered or spayed then he will be more likely to bite and scratch when feeling threatened. I’m sure you don’t want to get bit by an unneutered male cat!
And what about cats that are already living together? You should take some time to read this post on how to introduce two cats before taking them in your home together: http://www.raisingcatswithsuccesses.com/2010/06/how-to-introduce-two-cats.html
How do I introduce my new kitten into the family after all these years? How do you integrate your new cat into your existing life with your current felines? Here are some things worth considering :http://www.ehowitworksdaily.com/newly-adoptedcat/?utm_source=feedburner&utm_medium=email&utm_campaign=Feed%3A+EHOWITWDaily+%28EHOWITDaily%29