Bandits or Raiders are humans who will attack other survivors. Their motives are various, and can include
A bandit from "The Walking Dead" video game by Telltale
anything such as theft, murder, or general harassment. Bandits can vary from loose groups of violent, wandering thieves to highly organized groups and syndicates. Bandits' backgrounds can also vary. They could be anything from civilians desperate for supplies to rogue military units who feel free to take what they want due to their military status and superior firepower. The relationships between members of bandit groups can vary from family, to close friends, to mere acquaintences, or even to survivors who hate each other but need to stick together in order to survive.
Survival is the main objective. As they may outnumber and out-gun you, one should use discretion and pick battles wisely.
Encountering
Bandits are known to carry various types of weapons such as blades and firearms. Bandits are both armed and usually in groups, so they should be avoided at all costs. If forced to engage be careful to remain hidden as long as possible. Bandits may be enticed to targets that have signs asking for help from the government. Surprise attacks and traps should be used. Never attack unless absolutely necessary. Remember, they rarely work alone and have better experience fighting humans. Shoot to kill, then move any corpses as far away as possible and try to make it look like a zombie attack in case their friends come looking. Be prepared to defend your base or move out at a moment's notice. It is best to be very careful when allowing newcomers into your base as they may be bandits waiting for the right moment to make their move.
Your base or traveling group should at least look intimidating. Keep psychological weapons such as rocket launchers, machine guns and armed vehicles and make them clearly visible in locations where they cannot be stolen. This technique does generally work, but may attract bandits. Any bandit, no matter how large their group may be, will seriously reconsider attacking you if there is the possibility of having to face down a tank. But be careful, for you placing such items where bandits can see them might attract them. Also beware, for bandits can travel in great numbers.
However, if you are outnumbered, outgunned, and outmatched, it is a good decision to avoid the bandits.
Travel in groups and go heavily armed into unfamiliar or unsafe territory. Areas that are not thick in zombie activity are good candidates for bandits (the most vulnerable groups would likely move through these "safer" areas to avoid attacks from zombies) so do not drop your guard. A raid on a group or base is an activity that can use up precious resources, so most bandits would not commit to an attack if they think the reward is not worth the risk.
You can spot several characteristics and traits of a new group of survivors at a distance if they have yet to notice you for you to decide whether or not to approach them. Bandit groups are usually alpha male dominated and heavily armed (could be both with melee and ranged weapons). They are sometimes not very well organised which often results in them arguing with each other over supplies or other minor issues. They may also engage in questionable anti-social behaviour such as cannibalism, rape, torture, involuntary euthanasia, using drugs, keeping the zombies for non-scientific purposes e.g. "as pets" and other unethical practices due to the lawlessness aftermath of the zombie apocalypse.
You should avoid such survivor groups if they meet some or all of the characteristics mentioned above.