I agree. But the fact that we even question those tactics like you just did proves that we think about it differently than ancient, pre-Christian societies. I can't think of an ancient text that questions what we would now think of as crimes of war.
For example, Caesar committed genocide on a massive scale in Gaul--killing up to a million people (1/6 the estimated population) and enslaving about an equal amount. Yet, I think most historians believe the only questioning of Caesar's actions were from jealous Senators and elites who didn't want him hogging the glory or the riches and were afraid of how popular he was becoming among the plebs. But even they only questioned the legality of his authority to push so far, not the wiping out of entire tribes, etc.
DANC, let me recommend
Dominion to you, by Tom Holland. It's an interesting work of history, comparing the various ideas of the ancient world to Christianity and showing just how radically different they are. And Holland is not a Christian apologist, by any means.
It is long, though. If you want something shorter, you can just listen to a bunch of his interviews (because podcasts rock!). Here's one (ignore Holland's clothes; he's smart and very educated, but a terrible dresser):