I tend to assume that people and animals in dreams represent parts of ourselves. What are your associations to cats? Do you think they are particularly cute? Loving and lovable? Loyal? How about black cats? Scary in some way?

It seems like some kind of mercy killing, killing to save the cat from future suffering. Do you have any idea about what you expected would happen to the cat if you left it alive?

Note that this is a very resilient cat. He can't be killed and in fact can't even be turned against you--he continues to show affection. You might consider if some part of you is very strong and resilient and perhaps you need to acknowledge that part.