Someone who's dastardly is a scoundrel or a cowardly villain. A superhero might try to protect a city from a dastardly criminal mastermind.
Use the adjective dastardly to describe a person or action that's underhanded and mean. Criminals, pirates, and bullies are all frequently called dastardly. An unprovoked military strike is also sometimes described as dastardly, referencing President Franklin Roosevelt's famous 1941 speech after the Japanese attack on Pearl Harbor, which he called an "unprovoked and dastardly attack by Japan." In the 1500s, dastardly meant "dull," from the now-obsolete dastard, "one who is lazy or dull."