Why was Idi Amin never brought to justice by the international community despite the atrocities he did?
Didn't UN or other international agencies think it is appropriate to bring Idi Amin to justice while he lived peacefully in exile after all the atrocities he did in Uganda?
Answer by Balaji Viswanathan:
Kill one person, you go to prison; kill 20, you go to an insane asylum; kill ten thousand, and you get invited to a peace conference
Despots live happily
It is not just Idi Amin. A big chunk of world's despots are never brought to justice. No one punished Stalin for what he did. Same for Fidel Castro and the lunatics who have ruled North Korea. The butcher of Cambodia –– who has done among the most attrocities in South East Asia just got a rap on the wrist. He died in his home peacefully.
After having a long reign of terror in Paraguay,happily retired in Brazil. Under Ethiopia was thrown to the vultures and the guy happily wrote memoirs about it. He has been sentenced to death in absentia but spends happy times with despot bro who was once busy printing 100 trillion dollar notes. Haitian dictator, the "Baby doc" who tortured and killed thousands, spent quality times among the vineyards and cheese factories of France. Of course, killing is such a stressful thing and these guys need such vacations to rejunevate.
Why does this happen?
Until the Second World War, no one was tried for "war crimes". The losers in the second world war were tried for war crimes, although no one from the winning side was put through the ignominy. In the case of Nazis and Imperial Japan armed forces, they had surrendered before the Allies and thus it was easy to try them.
Internal strife and even civil wars are still largely outside the parameters of war crimes and the grave breaches provisions of the Geneva conventions. 
Now, how would you bring a random despot ruling Cuba to justice? Unless there is a war that the West is actively involved, they would never have enough incentives to bring a criminal justice. There are way too many logistical and motivational issues. The proverbial East is too stuck in its own problems and don't want to bring attention to them.
Idi Amin didn't matter a lot to the West. After all, most of whom he affected was Asians and Africans. And he got a really powerful protector in Saudi Arabia, who is really important to Europe and the US. National interest always trumps moral obligations.