The elder Hamlet acts ethically in all things, even in war. As evidence, see Act 1.1.95-100. Horatio is reminding Marcellus of this fact after the mute appearance of the ghost of the King. The king, "who by a sealed compact/Well ratified by law and heraldry,/Did forfeit, with all his life, all (those) his lands/Which he stood seized of, to the conquerer." Horatio tells him that he would have acted in accordance with justice no matter who was the victor "Had he been vanquisher" (105).
Claudius, however, knows no rule of ethics at home or abroad. He will do whatever it takes to gain and keep power, including murdering his own brother for the throne. He is a "hyperion to a satyr," that is, the Sun God, full of light, compared to the demonic half-horse, half-man, who was known to be full of lust and cowardice.