A good or bad propaganda can be defined by what it aims at achieving and the methods used to achieve this.
Propaganda are usually carried out with the aim of influencing people's feelings and their actions. It can have good effects and bad effects.
A good propaganda is used to influence attitude and actions positively. Like in a scenario where the media is used to warn about the dangers of a new disease. A good propaganda appeals to the people logically as well as emotionally.
A bad propaganda is one that achieves it's purpose by driving conflict and controversy deliberately. It is used to bring change deceitfully in a democratic setting.
