It's fine to care about both human suffering and people who kill animals for fun. But people who have tweeted about Cecil the Lion and not Trayvon Martin need to fuckin' reflect on their values.
These things aren't about our values, they're about what's being fed to us via the media. If a baby falls down a well, people will forget about this lion. If then, an athlete deflates a football we will forget about the kid in the well, if then a cop shoots an unarmed black kid we will forget about the football and if then a woman cuts off her husbands penis and throws it in a field of grass we will forget about the black kid. Average American has no attention span. Media feeds us these things. I refuse to be fed. Wish more people did. Edit: actually these things are definitely about our values. "We" value entertainment more than information. Individual safety more than collective justice. We are often not our brothers keeper.
In addition to what TNG said, it's also helpful to recognize the various motivations behind the outrage for each incident. Reactions to Trayvon's slaying fell into several camps. The obvious are the polar opposites of terrified black folk protesting "It could have been me" and the vile and proudly racist "good riddance". In the middle however there was another "It could have been me". The people want to be able to protect their property with lethal force, but are scared shitless of fucking up in the process. They basically sided with Zimmerman because they would also like to walk if they incompetently protect their domain and kill a kid armed with skittles and iced tea. The outrage at Cecil's killing also falls into multiple camps. There's the varying degrees of genuine conservationist view, the entitled-to-kill-all-the-things-for-entertainment view, and then there's the view you're probably most seeing on twitter. It is probably best described as "I'm jealous that someone is able to spend the average middle class annual salary on a holiday" thinly veiled as conservationist. I am confident that the Trayvon "I might fuck up and kill a kid with skittles" crew and the Cecil "Fuck you for spending my entire annual salary on your safari" crew have a fairly broad overlap.
It is too bad that in America the middle is almost always lauded as "moderate" and the right way, as though it's normal and correct to be afraid all the time and think (wrongly) that guns will make your family safer and not way more likely to die from a gunshot wound. The class envy aspect of the Cecil thing is interesting, particularly if it's coming from people who vote against living wages and inheritance taxes. But what can be done.