Graffiti is considered bad because it is associated with broken window theory and other kinds of street crime. Graffiti is associated with gang activity and tagging behaviors whereby criminal groups indicate the areas they circulate by painting specific symbols on walls and other structures. Graffiti encourages littering, loitering and illegal behavior.
Graffiti is strongly associated with broken window theory, the idea in criminal justice and policing that states crime and delinquency are more likely in areas where signs of financial and social breakdown are apparent. Painting on walls and defacing buildings indicates a lack of police presence and adherence to social norms. As more signs of social dysfunction become accepted in certain areas, the instance of illegal activity increases, thereby making graffiti an indicator of troubled areas.
This style of using spray paint and other kinds of permanent paint on structures is also strongly tied to gang activity. Various gangs use different symbols they place on public edifices to indicate they are claiming these areas as their territory. Such displays result in increased crime and other types of violent events. Turf wars, gang shootings and other types of problems are strongly linked to graffiti, causing many to label it as a bad sign.