Whenever you see the prefix "mal-," you know it's not good. Malfeasance is bad behavior, especially from officials or people who should know better.
If nothing else, the mal- in malfeasance will alert you to the fact that something bad is going on. If you know French, fease-, faise- will ring a bell, as it often means "to do." You can probably then infer that malfeasance means "to do bad." Today, it's usually a bad deed done by an official or an organization. You wouldn't accuse a dog who peed on the carpet of malfeasance, but you would accuse a mayor who took a bribe of malfeasance.