Green’s Dictionary of Slang

(US) to shoot dead, to kill.

1990		G.R. Clark Words of the Vietnam War 451/1: Sanitizing of suspected VC or their supporters was routinely carried out by special assassination squads employed by the program. [...] When an individual was ‘fingered’ as a VC/VCS/VCI by an informant, a check into the accuracy of the informant’s information might not be done, and the fingered VC ‘sanitized.’.
2002		(con. 1967) J. Laurence Cat from Hué 442: Of all the words American troops used to describe death in Vietnam—aced, blown away, bought it, croaked, dinged, fucked up, greased, massaged, porked, stitched, sanitized, smoked, snuffed, terminated, waxed, wiped out, zapped—the one I heard most was ‘wasted.’.