Emotional approaches, including appeals to shame, probably carried a large part of the impact of the civil rights movement. But there's a difference between "I am a man, a human being with thoughts and aspirations, and I believe I have a right to be treated with dignity," and "what you're doing is evil, and any attempt to justify yourself is sheer moral imbecility."
Emphasizing the sheer moral weight of the arguments they're dismissing by arguing for death might encourage them to consider that they may be doing something wrong, but calling their arguments stupid, or far worse, evil, is likely to signal to them that they've been targeted as an enemy, and cause them to dig in their heels.
Last Wednesday (2010 Dec 01), BBC Radio 4 broadcast a studio discussion on the question: "should we actively try to extend life itself?" The programme can be listened to from the BBC here for one week from broadcast, and is also being repeated tomorrow (Saturday Dec 04) at 22:15 BST. (ETA: not BST, GMT.)
All of the dreadful arguments for why death is good came out. For uninteresting reasons I missed a few minutes here and there, but in what I heard, not one of the speakers on any side of the question said anything like, "This is a no-brainer! Death is evil. Disease is evil. The less of both we have, the better. There is nothing good about death, at all, and all the arguments to the contrary are moral imbecility."
Instead, I heard people saying that work on life extension is disrespectful to the old, that to prolong life would be like prolonging an opera, which has a certain natural size and shape, that the old are wise, so if we make them physically young then old people won't be old, so they won't be wise. Whatever cockeyed argument you can construct by scattering into a Deeply Wise template the words "old", "young", "wise", "decrepit", "healthy", "natural", "unnatural", "boredom", "inevitable", "denial", I heard worse.
If I can bear to listen again to the whole thing just to check I didn't miss anything important, I may write something on their discussion board.