I can link to some earlier answers about Roman imperial and expansionism specifically. The Romans generally used some justification for their brutal conquests, usually something like "protecting our allies" or "they attacked us first!". Recently this has been written about here by u/iguana_on_a_stick (with interesting contributions by u/Tiako) and here by u/MagratMakesTheTea, as well as some years ago by u/Astrogator in this thread. There are also two answers, this one by u/MichaelJTaylorPhD and this by our Iguana, on the sack of Carthage specifically