Use 'dastardly' to describe actions that are considered morally bad or evil by most people. The villain's dastardly scheme involved kidnapping the hero's sidekick.
Pro Tip 2/3
Sneaky Implication
'Dastardly' implies that the action was done in a sneaky or treacherous way, not just openly mean. He was fired for his dastardly manipulation of the company's finances.
Pro Tip 3/3
Old-Fashioned Tone
Although 'dastardly' is marked as modern usage, it may sound old-fashioned, resembling language from classic literature or films. The detective revealed the dastardly culprit with a flourish.