I think that Agamemnon's motivations are more in line with his opportunity to take Troy more than in showing loyalty to his brother.  Homer depicts Agamemnon as the type of leader who can seize a moment, manipulating an opportunity for his own benefit.  It is here where his motivations as a leader take over.  When Paris absconds with Helen, Agamemnon recognizes the moment as a chance to add more to his kingdom in engaging and defeating Troy. Agamemnon is able to use the offense committed to his brother as a justification or a prop for the larger motivation of being able capture Troy once and for all.  His motivations as a brother are secondary to his primary desires as a leader.  In this, Homer shows Agamemnon to be a king of grand visions and being able to see what can be from what is.  In doing so, Homer is able to show Agamemnon's motivations as being distinctly different from Menelaus, whose average tendencies would not be able to envision an opportunity in such a manner that Agamemnon does.  It is here in which his motivation as a leader supersede those of a brother.