According to this research paper, only 76 incidents have been documented as 'Halloween sadism' at the time of its writing (1985). Of those, two have resulted in death; however neither case turned out to be caused by candy received from trick-or-treating.
In 1970, five-year-old
Kevin Toston died after eating heroin supposedly hidden in his Halloween candy. While this story
received considerable publicity, newspapers gave less coverage to the follow-up report that Kevin
had found the heroin in his uncle’s home, not his treats (San Francisco Chronicle, 1970).
The second case:
In 1974, eight-year-old Timothy O’Bryan died after eating Halloween candy
contaminated with cyanide. Investigators concluded that his father had contaminated the treat (Grider. Sylvia, 1982 “The razor blades in the apple syndrome.”).
So in both cases, the deaths resulted from family member negligence or harm, and not some mysterious stranger handing out halloween death. I haven't found anything more recent yet.
From the synopsis at the beginning of the paper:
This paper examines the widespread belief that anonymous sadists give children
dangerous treats on Halloween. A review of news stories about Halloween sadism from
1958 to 1983 suggests that the threat has been greatly exaggerated. Halloween sadism
can be viewed as an urban legend, which emerged during the early 1970s to give
expression to growing fears about the safety of children, the danger of crime, and other
sources of social strain.