You won't be surprised to hear that I think people take shit way too seriously. If everyone just fucking relaxed a little, and stopped projecting negative intention onto everyone else, things would go a lot smoother.
I've been called out before for doing the opposite, which is assuming everyone's joking, but let me tell you, it makes for a happier life. And I get that some things aren't jokes, and some jokes are mean, and many kinds of speech can incite and encourage negative behaviour, and still I think we should assume the most positive intentions of one another. The contrary is this shit-mess we've got ourselves in right now.
If I'm trying to make someone laugh, nothing you say about the words I chose are going to change what my original goal was. Yes I might have said it better, or made a better joke, or taken something into account I hadn't thought of, but I was still trying to spread joy. Have we forgotten the value of that? Let alone the value of people who understand that there could be collateral damage, and make that joke anyway, knowing that there will be more positive than negative? I'd much rather have ten people laughing at a joke and three people pissed off, than not tell the joke at all. That's a plus seven turned into a zero by cowardice.
Some people think pissing off those three people isn't worth it. Sometimes, I might agree. But when you can be sure that at least one of those people has a stick up their ass, and another of those people is a moron, the numbers start to tip even further toward free speech and humour.
I think Joss Whedon was just trying to be funny when he tweeted that. Yes he doesn't like Paul Ryan, and yes the tweet was technically violent. Fuck it, let's even go so far as to say that he actually would enjoy it if a rhino fucked Paul Ryan to death, it's not like he thinks it will ever happen. And it's not like he's inciting rhino wranglers to unleash a horny one into Paul Ryan's private bathroom during tubby time.
Short Answer: Everybody chill the fuck out. People can say what they want. Even if it's not kinda funny. If we didn't have brave souls who said fucked up things, many people would never think differently, or laugh to help heal all the real problems in the world. Rhino fuckery isn't one of them.