The best thing I ever killed a PC with? Syphilis.
I also had a PC murdered by a Sweeney Todd-esque butcher once. The other PC's who were investigating the murder spree, and later also the mysterious disappearance of their friend, visited the murderer during their inquiries. As a gift the "jovial" butcher gave them sausages he had made from his "secret recipe". After they found who was the culprit the expressions on their faces was priceless. They kept asking for long time if their friend had been the secret ingredient, but as cruel DMs are wont to do, I never told them.