Aria's view both of herself and the Angels is morally relativistic. She sees the Angel Cartel in Curse as a fringe entity doing what it must to survive-- and, what's more, a necessary "ordering" force in the empire underworlds. As a shadow-society, it has its own rules and its own sense of right and wrong, which do in fact dictate the underworld's morality.
In other words, it's a suitable allegiance for an otherwise "orderly" person who cannot fit into an empire society's mainstream-- and a necessary limiting factor on the doings of more dangerous, genuinely destructive elements.
(The reality of these perceptions is, of course, open to debate.)
This may seem like a fringe view (Aria, after all, doesn't even see herself as human), but it might be fairly widespread, especially in societies where criminal organizations are an active tradition and enjoy a sort of social quasi-acceptance (see, e.g., the Yakuza, to cite a well-known example). The Cartel, or similar organizations, also likely enjoy similar perceptions wherever governments haven't done such a good job of maintaining order, thus delegating the responsibility to criminal gangs. The civilian population of the Sharkon system was actually in favor of recognizing the Angel Cartel, rather than the Minmatar Republic, as its governing authority for a time.