Didn't Rome save a lot of lives by pax romana? Perhaps it was actually better in terms of loss of life to be conquered by Rome than have endless wars among yourselves. Applies to the US too.

You can defend a lot of atrocities by arguing "for the greater good" and comparing to uchronic hypotheticals. I could as well argue that without Rome, the greek democracies would have been much more prevalent, and lead to modern democracies much sooner. Or that a world leader would have emerged, leading the ancient world to endless peace and prosperity.