Русско-англо-русский юридический словарь

If you cover your face with a bandanna, jump on your horse, and rob the passengers on a train, you're a bandit. A bandit typically belongs to a gang of bandits who commit crimes in remote, lawless, or out-of-the-way places. (Vocabulary.com)

He took part in sting operations to foil multi-million pound drug deals and has put several dangerous gangsters behind bars. (BBC)

A notorious Haitian gangster is due to appear in US court Wednesday for allegedly leading the armed abduction and kidnapping of 17 Christian missionaries last year. Joly Germine, 29, is said to be a leader of the 400 Mawozo gang, infamous for holding foreigners for ransom. (BBC News)

A gangster is a part of a (generally questionable) group, and may commit many different crimes. These often include theft, violence, or crimes involving illegal drugs.

Gangster: A gangster was arrested for selling illegal drugs and his involvement with a local gang.

Bandit: A bandit stole the women’s purse and ran into an alley.

Gangsters roll in more or less organized groups/"sets" usually and participate in serious crime, anywhere from thievery to murder. A bandit is a thief who is involved with a less intense criminal group. [...] However, when a person says "bandit", the mental picture is quite often someone stealing with a covered face.

For example, the Mafia are gangsters. The random guy robbing the gas station probably isn't. Robin Hood and his crew were bandits. (HiNative)