"Didn't want to" and "couldn't afford to" are two very different things.

As far as our "humanity" being defined by the way we treat "lower" animals, humans have been hunting those animals for food and survival for pretty much all of our existence. Even today, there are cultures in North America where, if you tell them you're a vegetarian, they simply won't understand the concept. To say that they're inhuman, or subhuman, because they don't subscribe to a modern philosophy (that is shared only by a tiny minority), is an attitude I cannot agree with.

Shackelton was in charge, he was responsible for the lives of his men, and he made a command decision. I don't see anything reprehensible in that. YMMV