Why predators kill, just to kill, without eating the prey is too complex to answer in a quick reply.
I have seen it in all mammal predators from feral cats to mountain lions. Even in the highly ordered wolf society- that's what got them in deep trouble with the cattle industry. No one minds a loss here or there if it is used in the food chain; but wanton waste is another issue.
Many times, the killer urinates or deficates on the uneaten carcass which indicates territoriality. Other times, they may "play" with the prey for hours before killing it. In my opinion, they often kill just because they have learned that they can.
We always do a stomach content on the wild predators we take/kill doing such damage, and more often than not, they are full to the point of regurgitating.
Feral or free-ranging domestic dogs & cats are a major threat to all wildlife as they will definately kill for "fun".
