I almost forgot, there is another side to this, I think.
Though it is inexcusable to make such comments or "jokes" around those who are sensitive to such issues, as a sufferer of ADD, depression, anxiety and sound sensitivities (the very severe kind, many noises can hurt me), I find that sometimes, it can make me feel better to make fun of, not the issues and how they affect others, but how I've experienced them. It actually makes me feel better. Of course, only with people who are comfortable with this.
Two months ago, I was near-suicidal. One of the first things I learned in therapy, was that it's ok to feel bad about things, and my therapist taught me to take things I'm not comfortable with about myself, and make them into things that make me, and my twisted little friends smile. I stopped being the downer at parties, and I became more of the comedian instead. It's been helping some of my anxious/depressed friends too, and I feel much more comfortable in my own skin now.
There is a big difference, however, between private conversations with others who relate and can get off of humor, and are not offended, and telling others to "kill themselves." I have a very dark sense of humor, though I make sure others are ok with it before I let it show, but of all the dark jokes I've ever made, telling others to commit suicide is one that I will not stand for. Even if it's joking among friends, it may make the friend seem like he/she's unwanted, and not really a friend. It's terrible to have no friends, but I think it's worse to feel like a fake friend. I know. I've been there. This person is a jerk. If someone is uncomfortable with a dark joke, they need to stop. There is no excuse for offending another on purpose.
Sorry this was more ranty than my last post. I've done some thinking, and I wanted to present another point of view. I'm ready for the hating now.