I think having your cat kill maybe two squirrels and a pigeon in some suburb isn't an environmental or moral catastrophe (HAW).
Doesn't matter, though, because our cat was found with a broken hip (in the wild, where he evilly killed small rodents and clearly didn't even eat them due to being evil, as evidenced by how skinny he was) and we don't let him outside unsupervised.
No, one cat killing a couple animals isn't. But outdoor cats usually kill at least twice a week, often more. When you have 30 or 40 cats in a suburb (not uncommon), that begins to add up.
It's estimated housecats kill about 4 billion birds in the US alone every single year, and they've contributed to putting several species on the endangered list or crippled their recovery. And birds aren't even the primary target for cats, making up a relatively small portion of their kill count (only 1 in 10). This is the same logic that says "It's fine for my to toss my trash out the window because a little trash isn't a big deal".
And no, of course cats aren't evil - likes sharks aren't evil. They simply have things they do. My brother has a cat, and I love the thing, but it's managed to escape twice, and killed several animals before he was recovered. If he was an outdoor cat, and didn't get bored, I think their neighborhood would be depopulated in a weak.
While it's a funny thing to joke about, cats really aren't sadistic or anything.
While cats aren't
evil, I wouldn't go that far. In my experience, they will specifically seek out these experiences - that's not just a reaction. The (very few) studies that have been done seem to indicate they gain pleasure from the experience, judging by the chemicals in the brain. Not definitive, but pleasure IS one of the ways evolution reinforces instinct. (Look at sex)
It's certainly conceivably cats could be hardwired towards sadism in certain situations.