Bulger was an opportunist that played both ends against the middle. As for being a rat, is it more honorable to be a killer, thief, or extortionist than to be a rat?
Basically Bulger was an extortionist or protection racketeer who made money shaking down small time criminals for "protection money", under threat that they would be physically harmed or ratted out to LE if such monies were not forthcoming. Because of his street rep for using extreme violence on non-cooperators, most of his petty criminal victims complied.