It's powerful (dramatically more powerful than a .22), relatively light, and can be loaded in high capacity magazines.

Just to be clear... I'm not saying that you can't kill somebody with a firearm chambered for .223. Of course it's potentially lethal. Pretty much all firearm rounds are potentially lethal, even something like .22 Short rounds.

What I'm saying, is that the .223 is not particularly more dangerous than other common rounds, to the point that there's any reason to single it out for special attention. And the fact that military forces choose it doesn't dispute that. There are a LOT of reasons why military forces make the choices they do, and they're as likely to be economic forces as purely strategic ones.

When the military chooses it as the round of choice for killing people, it's pretty nonsensical to try to hold it as some sort of weakling.

If sheer lethal effectiveness were the only criteria used to select a round, .223 would not be the first choice for killing a human being.

Of course you can make any point with relative comparisons. Compared to a bb and a slingshot, .223 is deadly-as-fuck. But compared to many other rounds that you can commonly find firearms chambered for, it's average at best.