
Dastardly
Negative Connotation
Use 'dastardly' to describe actions that are considered morally bad or evil by most people. 
Sneaky Implication
'Dastardly' implies that the action was done in a sneaky or treacherous way, not just openly mean. 
Old-Fashioned Tone
Although 'dastardly' is marked as modern usage, it may sound old-fashioned, resembling language from classic literature or films. 