According to knowledgeable sources including former CIA operatives and researchers this is pure myth. For instance former CIA agent Bob Baer (the basis for the george Clooney character in Syriana) says "If that were possible don't you think we would have done it in Vietnam? Programed VC prisoners to go kill there generals.
Back in the 50s the CIA did experiment with mind altering drugs to see if they could control human behavior but the experiment didn't work out very well. The test subjects became more suicidal than homicidal. This according to now declassified CIA documents.
It made for a great movie plot and for some entertaining conspiracy theories but there is zero evidence that a Manchurian Candidate (programed assasin) has ever been created.
For me anyway this urban legend has been debunked.

