I completely hear you. My dad has Alzheimer's. I would never take him on a cruise, simply because of it (he doesn't do well when he's not at home in his comfortable, safe environment), but quite often, he gets confused, and when he gets confused, he gets angry. He was NEVER an angry person when I was growing up. I NEVER saw his temper! But now, because of this disease, he definitely has changed personalities.
Again, I'd never take him on a cruise ship because that just wouldn't go well for ANY of us! NONE of us would enjoy that trip! But other families might want to take that relative on one last cruise before they pass on. And I can see that too.
I completely agree with the "hidden illness" thing. For YEARS, nobody would have known that my dad had any sort of senior memory problem (now, it's impossible NOT to know, but he's had this for many, many years) unless they were with him for extended periods of time. So passing strangers might have thought that he was perfectly fine mentally, but was just choosing to throw a fit at some point in time. When in reality, he couldn't really control himself in that manner anymore. He lost that ability to reason and to know what's going on.
So while I don't defend their actions, and again, I personally would never take my dad on a cruise ship or any extended vacation BECAUSE of his illness, some elderly people simply can't help it. They don't have the mental capacity to control themselves anymore. Sad, yes. But also very true.