A 1909 newspaper article describing the killing of a man by his own son was headlined ”The Deed of a Dastard.” I’ve never before heard of this word used as a noun. Makes sense I suppose.
AHD, however, lists dastard and dastardly as separate entries rather than dastardly as the adjective of the noun listed in its “Other forms” category. Curious.
Etymonline doesn’t have a separate entry for the noun form, but notes that the noun hails from about 1440 and the adjective is first cited in 1567.
This site offers the interesting possibility (though offered as if it is fact) that dastard “is widely avoided” because of its “fraternal twin” that begins with a “b”. Think so?