Hydrogen cyanide has been used in prison executions. It's also found in cigarette smoke.
"Smoking and Tobacco Control." U.S. Department of Health and Human Services, Public Health Service, National Institutes of Health, National Cancer Institute. Risks Associated with Smoking Cigarettes with Low Machine-Measured Yields of Tar and Nicotine, 19 Nov. 2001. 56, 176, 251.http://cancercontrol.cancer.gov/tcrb/monographs/13/m13_complete.pdf