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but put a person on a bike, and no one gives a fuck if they get killed

Give an animal a name, and people suddenly give a shit. shocking

I'm sure that's an important factor, as is a certain awareness of the conservation status of much African megafauna, and 'The Lion King' etc., but I suspect the photographs of the lion looking magnificent and lion-y would be the main factor. Then there's this:

http://www.theguardian.com/commentisfree/2015/jul/30/the-guardian-view-on-cecil-the-lion-the-immorality-is-in-the-pleasure-of-the-kill

A very large number of people are sensitive to such obvious displays of cruelty and, as the Grauniad says, taking pleasure in cruelty, while far fewer are sensitised to the cruelty involved in most animal husbandry (not that I would want anyone to be sensitised to that, as it's horrid). As another example, the political case against animal experimentation is still much stronger than that against factory farming.

Some people with a masculinity complex dope and 'win' Tours de France, others kill lions with the help of modern technology. It's the same thing in essence, you use tools to enhance yourself and fool yourself into thinking that you're more than you are.