Before the dogs are used to hunt, they are starved to make them hungry for the prey.
Good hunting dogs are "rewarded" by not being violently slain, and they are still often kept chained in stinking "zulos," underground bunkers soiled with feces.
When the dogs reach two or three years of age and are weakened by malnutrition and lack of care, it is simply cheaper for the galgueros to kill the dog to avoid feeding the dog until the next season, when they simply pick up a new dog for ten euros from one of many breeding facilities that supply the hunters. This is much less than the cost of food to maintain an adult dog between hunting seasons.
Because the galgos are regarded under Spanish law as working dogs, they are excluded from the laws relating to pets. They are considered to be goods, no different than agricultural machines that can be disposed of or used in whatever manner their owner decides.
Good dogs, the ones who have not shamed their owner by being poor hunting dogs, are "rewarded" by being sent to perreras. These are municipal facilities where the dogs can be euthanized.