"Life is life - whether in a cat, or dog or man. There is no difference there between a cat or a man. The idea of difference is a human conception for man's own advantage." Sri Aurobindo.
Racing greyhounds is an animals-as-entertainment business. Driven by a profit-focused gambling industry, dogs are bred for the purpose of racing – and are expected to be fast in order to be profitable.
But not all of them will have what it takes. When the dogs don't make the grade, are burnt out, will not or cannot comply, most of them are discarded and destroyed.