People who can't speak, are fed through tubes, get around on wheels, express emotion in nonstandard ways, lack functioning hands, and can't have most forms of sex, don't usually want to die

Except when they do.

You might be a very atypical person who'd prefer death to severe disability, but if you are, could you pepper statements like that with disclaimers? That's kind of a dangerous meme to reinforce.

Tony Nicklinson's case is by no means the only one I've heard of. How do you know that these people are "very atypical" of the severely disabled?

Of course, the idea does lend itself to rationalisations, and according to this blog post, Ken Wood, who you quoted, is doing exactly that:

This view contrasts sharply with the reality that most of the patients killed were not particularly debilitated and perpetrator Cathy Wood’s own statement that “we did it because it was fun” (quoted in Cauffiel, 1992, p. 254).