Especially when you're doing research on the nastiest and messiest and quickest way to kill off a character with cancer. Only it can't be too nasty, or it would only take a few weeks and we don't want that, but it can't be too survivable, so that the reader won't buy the quick disease progression. I'm usually able to maintain a very healthy emotional distance when reading about medical stuff (a book on the plague in Europe comes out for Kindle in three days, and I will be purchasing it), but having lost more family members to cancer than anything else...well, come to think of it, that's about all that takes us out (knock wood!). Suppose it could be worse.
Lori Moore once came to talk to my MFA workshop about getting a book called A Writer's Guide to Poisons. She was practically giddy about it, but I think that a large part of that giddiness was her enjoyment of the effect that telling people usually had. I don't know whether she expected us to pull our drinks in a bit closer; personally, my reaction was to check out that book on Amazon as soon as I got back to my apartment. (There was also the moment when I got a beautifully shocked reaction out of her when I told her I usually stay away from caffeine, but that's another story.)
All that said, I do still feel just a little guilty as I plan a fictional character's death. Unlike me, they never really see it coming.