Gnolls, Grimlocks, Lizardfolk, Locathah, and Troglodytes in core.
Rebuke as a 12th level (Improved Turning Bonus Feat) And any successful rebukes count as commands. And slightly better than 2 HD warriors, 24 hitpoints guaranteed, Natural Armor and Dex bonus.
I've always considered Power Word: Kill as a finishing move. The fight with that dragon has been carrying on for a while and you're starting to run low on resources, it's useful to just end it. No Save. And it's one of the Iconic Death Knight powers.
Makes sense, Fixed.
The basic difference, when I was designing them, between going straight KoD and taking the prestige class is the KoD tries to get in a Duel with an isolated opponent while the LotDL tries to slaughter entire populations. Just wipe towns off the map.