I'll help things that aren't becoming an active part of the food chain.
I certainly wouldn't get between a turtle and a shark... for a number of reasons. If a shark takes a bite out of the turtle and the turtle makes it to shore, thus the shark has been deprived dinner, THEN I may help the turtle, but mid-feed, nope. If the shark can get the turtle, then it's the shark's dinner.
And if something attacks *me*, I will beat on it with every available resource I've got and not feel in anyway unnatural or hypocritical. Having a human fight off a shark or a bear or a human's friend fight off a shark or a bear is perfectly natural since few species take a kind view of being eaten, and social animals (like humans) tend to defend each other. What would be unnatural is if while a human was fighting off a bear, a shark in specialized gear ran up on land and attempted to separate the bear from its intended prey because it found the human cute.