Dumbing it down

[Essex]

Davem, sorry but your point re the Nazgul

Therefore only supernatural means may defeat them.

Why? Is there a Fantasy Equation out there that specifies this?

Let's forget the books for a second, again.

Movie wise, Saruman was killed by a blade and falling onto a Spike. He, being a Wizard, shows to the average non book reading movie goer that 'higher' beings can be killed by conventional means. Therefore the moive Witch King can also be killed this way. So, to me, there is no problem in Merry / Eowyn killing him with conventional weapons.