Killing or ganking blue players in a lawful area will result in you losing alignment, and if your alignment drops below 0 you will become "red". Performing any negative action on a blue player in a lawful area can also flag you "rogue" or "grey" for a short duration. The same thing occur for players who help other reds or greys in these same lawful areas.
For those who accidentally go red or wish to change their ways and go back blue, you can gain alignment by killing monsters inside dungeons which are located throughout the world. You can also regain alignment by praying at altars found throughout the world. This costs 2000 gold to regain 20 alignment with a 11 hour cooldown.