We had a cat once who, we found out, liked to sit in the berry bushes in the garden, wait for a bird to land, and then kill them. She didn't didn't eat them - she much preferred the cat food she got. She just murdered bird after bird and left them to rot.
Cats certainly are not the best examples of harmony.
If not, why you think it's okay to condemn another living creature to such a life sentence? After all, its well-being is just as important as yours, is it not so?
Cats seem pretty alright with "lifetime seclusion", anyways. As with other domesticated pets, the only reason they exist is because they are, on the whole, happy being domesticated; if they all hated their lives (in a way that we were aware of), we wouldn't breed them to begin with.
(Apart from the scum that are some humans...)
Obviously not. All kinds of animals kill. Also it's not "for fun" in the way we're having fun. They don't plan on entertaining themselves that way, weighing and ignoring the moral impact, etc.
It's an instinct they have. It's training to hone their hunting skills -- which they need evolutionary to survive -- even if they we take there of them for the last 5-10.000 years (which is very little in evolutionary terms)
Since it seems to me that "fun" or "malice" or other motivations cannot be measured that precisely in animal kingdom, i think it is fair to say that surplus killing covers your question.
It's not, it's just their nature, which in other words is what they evolved to do.