This quote relates to the book Of Mice and Men. In this book, Candy is an old man who has spent his whole life working on a ranch. However, he lost his hand and he has no money now. Candy is extremely aware of the terrible fate that awaits him, as he has no hope for his future. The only thing that makes him happy is his dog.
However, much like himself, his dog is old and tired. Because he is no longer useful, people belive that he should be killed, but Candy cannot bear the idea. In this comment by Slim, Slim claims that the dog should die becase it is "old" and a "cripple," and thus there is no point in him living. This is especially insensitive towards Candy because Candy is also old and injured, which means that he is equally dispensable in the eyes of others.
