An important difference: The shooters of those 8 people are either considered criminals, and are eligible for prison, or there are extenuating circumstances (e.g. a shooting in self defense). So (with one notable exception) we feel like we are already doing as much as we can about this. But the people who shot the gorilla are not in any way culpable or held responsible. There won't be an investigation or a trial. I think this is why people feel like they have to speak out about this. Not that an investigation or trial is necessarily appropriate; but without any public censure at all, then the next decision to shoot an animal may be taken more lightly.
Also, it's easy to say that this gorilla was an animal, so who cares. But there are big differences between species. A gorilla has the same legal rights as, say, a rat, even though a gorilla is much closer to a person than a rat in terms of psychological sophistication.
The notable exception I mentioned above is the case where police shoot black people with impunity. Well, police in general do whatever they want with impunity, but this is one case where people feel like they have to speak out, and the reason is not that shooting black people is somehow worse than any other police shootings, but that there doesn't seem to be any institutional system in place to deter racist police from shooting black people. In other words, it feels like we are not doing enough.
Why is a lesbian a "bad role model"? Are there still people who think that you can decide to be gay? And even if you do, what is the problem with it? Many children have lesbians in their families and many of them are actually going to be lesbians (whether they see it on TV or not), or already know that they are lesbians. It would be nice if there were lesbian characters portrayed in films so that they know that it's accepted and relatively common and they don't have to feel the self-hatred and shame that you and other bigots seem to want them to feel.
That said, of course Disney shouldn't feel like they "have" to do this.