https://en.wikipedia.org/wiki/Surplus_killing
A local rancher lost a number of sheep to a mountain lion that apparently enjoyed surplus killing. After the first spree, the cat was captured, collared, and relocated but it came back and took out 11 more sheep plus an alpaca, after which it was culled by the state. Big cats are certainly interesting creatures, but it would be foolish to assume a given cat would care for something, eat it, or kill it for sport. They are all possible outcomes, and depending upon one's perception of cat psychology is a good way to get Roy Horn'ed and end up in a wheelchair.
The Wikipedia page also has weak evidence for lions being involved in surplus killing. It has two footnotes. http://www.aimspress.com/article/10.3934/mbe.2018031 is a simulation, and the abstract (article is paywalled) of https://zslpublications.onlinelibrary.wiley.com/doi/abs/10.1... only mentions “foxes, spotted hyenas and other carnivores”, and hints it may only happen when prey is easily available (as with a fox in a hen house, I suppose)
I think neither article claims it happens regularly.
What a simplistic view of lions. I agree that lions don't really have the capacity to domesticate a flock of sheep, but to suggest that they're mindless killing machines that would meaninglessly mass slaughter sheep the first chance they got is weird. They're complicated creatures with rich emotional lives, and I've personally seen them plenty of times pass up opportunities to kill because there was no point. Interestingly, there's also lots of youtube footage of lions and other big cats attempting to care for other animals (albeit typically ineptly).