Usually associated with gentlemen from the Quincy, MA area, calling someone a maggot can either be derogatory or congratulatory. Calling your buddy a "maggot" after hearing of his/her weekend exploits is a way of acknowledging that he or she had successfully waged a battle in the war on decency. On the other hand, calling someone a "maggot" for hanging out at the Quincy Center train stop conveys the total lack of respect that you have for the individual in question. As you can see, to an outsider this term could be confusing and hard to grasp at first, but to the people involved, calling someone a "maggot" is second nature.
"While the rest of the party was dancing and discussing philosophy, the maggot was slowly making his way towards the poorly hidden liquor cabinet, ready to fight for his feed."
"Ked, you're a fucking maaaaagot! I can't believe you smashed my girlfriend!"