It's not evil... They aren't trying to kill their pet. They obviously didn't realize there was something wrong with the food.
In all fairness, cats are well known for their fickleness about food. One day they love a brand or flavor, the next they refuse to touch it.
My current Korat fellow likes his food a bit too well for that nonsense, but the 2 tabbies I had before him would do things like that every so often and I wouldn't have thought anything of it...just them doing their thing again.
Doesn't excuse the dogs, but I can understand with the cats.