For Argentinians, it's a matter of pride. They argue that it was an unjust war. Argentinians were willing to send their sons to die for their country; society mobilized to bid farewell to the hero-men marching to the front. The media fueled optimism and triumphalist hopes among the population. But that was during the era of military dictatorship, so everything turned out horribly, with hundreds of young people left abandoned.
In the end, it was an unjust war because it was initiated by a military government. The fact that it was such a government was surely an aggravating factor, but it wasn't what constituted the injustice of the war itself. In reality, not even the most impeccable of democracies has the right to wage war. According to the UN Charter, states can only use violence against another state (a) in self-defense against aggression and (b) to enforce international law if they have authorization from the UN itself.