I suspect the Unbreakable Vow is being parsed here as adding high-level terms to someone's utility function, and that that's being interpreted as equivalent to erasing the previous personality.

I'm not so convinced, myself, neither that that's the right way to look at the spell nor that values are that tightly linked to... not sure what I want to call it. Personhood? Unique agency? Whatever we actually care about when we object to murder, anyway.

EDIT: Never mind, after looking through a page or so of DanielLC's comments I think that sentence actually expands to "[presumptively] giving people the death penalty [for breaking their Vow] with no way to ask for [...] exemption[s..., etc]." Pretty sure that's not how the Vow works in Eliezer's world, though, after reading the bit where Harry undergoes it.