I'll try to put this as simply as possible!
The difference between us and them is that they have the power to break up families and influence medical treatment. We don't have that power, and nor (I hope) do we want it. Any hate language they use against us is to try and reinforce that position. Any language we use against them is ultimately out of frustration at how disproportionate their power is.
The mentally diseased thing IS a big deal, because it represents a shift in the language being used by the Society. Put simply, they are getting sloppy. Look up "mentally diseased" in your Watchtower library, and you will see that it has been used frequently in the past, but always in context with the rest of the verse, i.e. "mentally diseased over questionings and debates" etc. To my knowledge, this is the first time they used the expression and applied it directly as a "diagnosis" against apostates.
Why do we care? I can't speak for any one else, but I personally care because I have family trapped in this wicked corrupt organization and I want to get them out. I will bring down the whole oganization, or severely undermine its influence, if I have to. If no one else wanted to join me I would do it by myself, but I get the feeling I'm not the only one that wants to do something.