How is feeding an aro a mouse which is a small mammal and entirely likely apart of their natural diet worse then feedin them a market prawn which they will not run across in nature? unless you are feeding them FW market shrimp? I've seen northern pike and musky take down juvinile ducks/geese and LM bass tag snakes/shrew/ect. imo any top water predetors diet does consist of mammalian prey. as long as it is part of a balanced feeding regeime it is not unhealthy. and likely provides some key nutrients for the fish. Which yes can or may be found in many prepared diets. But many predetors of said type can be very hard to convert to such diets. which IMO is cruel to starve your fish to get it to eat because you as it's owner refuse to offer it what it's natural prey would be.
and feeding a fish a live mouse is no crueler then feeding it a live fish, live worm, or anyother live food. now tormenting said food before feeding it is cruel, the act of feeding it in itself is not imo. I've watched snakes eat live chickens, piglets, and goats, let alone mice, rats,rabbits ect. The only cruelty in any of it is if the food is left to suffer/or abused prior to feeding. though I did lose a red-tail boa to a particularly viciouse rat. which was methodically executed (imo humane as it was a quick and as painfree a death as I could make) and fed to another snake.
people will always agree to disagree with feeding live. imo if its such a horrible thing. Go stake out an animal testing faciity where you will see true cruelty to any of the above mentioned animals.
a side note on the feeding of toads, beware of the toxins some frogs and toads produce. likely harmless in small quantities but not likely healthy in larger doses (ie, multiple feeding of strickly said animals)