Neutral:
* Offenders
* Lawbreakers
* Delinquents
* Transgressors
* Perpetrators
* Wrongdoers
Negative:
* Miscreants
* Villains
* Thugs
* Scoundrels
* Crooks
* Malefactors
* Outlaws
More specific:
* Felons (for those convicted of serious crimes)
* Convicts (those who have been found guilty in a court of law)
* Inmates (people serving time in prison)
* Gangsters (organized criminals)
* Terrorists (criminals who use violence for political or ideological reasons)
The best word to use will depend on the specific context of your writing.