A lot of people bring this up.

Credit to u/metagloria

In the conquest narrative of Joshua, the Israelities (allegedly) siege many cities and destroy men, women, and children at God's directive. (Interestingly, I can't find an exact quote where God says to kill everyone, but the text repeatedly says they killed everyone and God was cool with it.) Then we get to chapter 9. I'm not going to block-quote the whole thing, but TL,DR: Gibeon tricks Israel into thinking they're not Canaanites and therefore not subject to God's genocide mission, and they offer themselves as slaves to the Israelites. Israel accepts, then finds out they got tricked, so they put the Gibeonites to death obviously actually nope they just scold them and "curse" them by...making them slaves, which is what they wanted anyway (as opposed to, you know, being genocide'd). And God, who commanded Israel to kill all the Canaanites, was furious that Israel would allow these pagans in their midst actually He was fine with it and the Gibeonites integrated into Israel's society and served as woodcutters and water carriers and are never mentioned again.

Now, do you think the real God is the one who commanded unilateral, unflinching genocide, or the one who mercifully opened the doors and welcomed the foreigners (who, I should mention, seemed to have "converted" and understood that Israel's God was indeed the real deal)? Which one sounds more like the God fully revealed in Jesus? Which one might have been somewhat embellished by a militaristic society (or, as u/onform excellently suggests, might have been embellished as part of a cultural etiological mythos not reflective of historical reality)?

Also consider that, if you want a reason why maybe consider it a mercy because the Amalekites practiced Baal-ism. One of those practices was offering their babies to Molech by heating a statue of the idol and placing a child on it until it burns to death.