[Avodah] Questions about mechiyas amalek

[Ben Waxman via Avodah]

On 9/5/2017 5:01 PM, Zev Sero via Avodah wrote:
> David did; Yoav killed all the males but didn't know that the females 
> must be killed as well.

How is it that Yoav made such a grievous mistake? No one told him what 
the mitzvah and the dinim were?

Having said that, it seems that even righteous kings made grievous 
mistakes in the halacha. David (according to the Ramban) didn't know 
that counting the number of Bnei Yisrael was assur, and apparently no 
one informed him.

Ben