Well I'm not sworn, not yet, that is, but why is it, that to be sworn, you must play as undead? There are at least two (possibly 3 or more) examples, where it isn't the case.
Let's count, shall we?
1.
Almira, the Succubus - yes, she's a demon of sorts, but pretty much alive

2.
Sadha, the Red Princess - she's at no point in the game undead


3. Possibly
Lord Linder Kemm - he doesn't appear undead, when you meet him first in the Magisters barracs, nor is he, when you fight him in his vault to protect Lord Arhu. And yes, he's definitely sworn, as Windego revealed


And of course, there's
Windego, the witch - yes, after you fight her for the first time, she'll become undead, but thanks to your actions, when she's on board the Merryweather, she's alive and already sworn


There might be even more, that I've overlooked, or forgot about.
So why is there that requirement to be undead? Why?