The issue regarding the death of the Witch-king and who should be credited with the 'kill' is controversial with two decidedly different opinions. Some believe that Merry should be credited as having dealt the deciding blow, while others believe it should be Eowyn as helped by Merry.
The differences of opinion are based on the interpretation of the encounter described in "The Return of the King":
"Out of the wreck rose a Black Rider, tall and threatening, towering above her. With a cry of hatred that stung the very ears like venom he let fall his mace. Her shield was shivered in many pieces, and her arm was broken; she stumbled to her knees. He bent over her like a cloud, and his eyes glittered; he raised his mace to kill. |
But suddenly he too stumbled foward with a cry of bitter pain, and his stroke went wide, driving into the ground. Merry's sword had stabbed him from behind, shearing through the black mantle, and passing up beneath the hauberk had pierced the sinew behind the mighty knee.
Eowyn! Eowyn! cried Merry. Then tottering, struggling up, with her last strength she drove her sword between crown and mantle, as great shoulders bowed before her. The sword broke sparkling into many shards. The crown rolled away with a clang. Eowyn fell forward upon her fallen foe. But lo! the mantle and hauberk were empty. Shapeless they lay now on the ground, torn and tumbled; and a cry went up into shuddering air, and faded to a shrill wailing, passing with the wind, a voice bodiless and thin that died, and was swallowed up, and was never heard again in that age of this world."
So passed the sword of the Barrow-downs, work of the Westernesse. But glad would he have been to know its fate who wrought it slowly long ago in the North-kingdom when the Dunedain were young, and chief among their foes was the dread realm of Angmar and its sorcerer king. No other blade, not though mightier hands had wielded it, would have dealt that foe a wound so bitter, cleaving the undead flesh, breaking the spell that knit his unseen sinews to his will.
Those who believe that Merry is the one who strikes the deciding and mortal blow assert that the words "no blade" and "so bitter" implies killing the Witch-king. But in the context of the whole encounter it does not stand up.
From a story standpoint, Eowyn's character is gripped by frustration and despair at her situation, and so it makes no sense for Tolkien to have built her character up only to deal an empty and meaningless blow. Whereas the role of the Hobbits - especially Merry and Pippin is one of assisting and helping. But look at the specifics of the encounter and the words used by Tolkien.
When Merry makes his stab at the Witch-king, his cry is not one of mortal pain or a fatal cry - but a cry of bitter pain. And the result of Merry's wound is for the Witch-king to 'stumble' - *just as* Eowyn did to his blow to her as Tolkien used the exact same word for each - so it is hard to conceive that Merry's blow is one of death.
And what of the passage about the blade from the Barrow-downs? Well, look at the description of what kind of wound he dealt - "so bitter" - again the words used are far from being mortal or deadly or fatal. Also, the nature of Witch-king needs to be considered - he is of 'undead flesh' - he has eyes and skin and muscles(synonomous with 'sinew'), but they are unseen. The effect of Merry's blade is therefore completely localized - meaning he dealt a really painful (read as bitter) wound to the Witch-king's knee which made him cry out and stumble.
There is nothing fatal or deadly implied in this wound, whereas Eowyn's blow is the deciding one - see what kind of cry results? It rises up into the air as his body vanishes leaving the shapeless cloak behind - precisely because her blow, to the unseen and undead "flesh" is a fatal one - it is to a vital spot - a truly mortal blow.
Look at the words used by Tolkien to describe this - the Witch-king is said to be Eowyn's fallen foe, not Merry's.
And we also have substantiating remarks from other sources as well:
From LETTERS there is a most remarkable passage. It had to do with Tolkien's criticism of someone's feeble attempt at an animated screen play for LotR.
He is commenting on the fight with the Hobbits and Nazgul on Weathertop:
|"Aragorn did not `sing the song of Gil-galad'. Naturally: it was quite inappropriate, since it told of the defeat of the Elven-king by the Enemy. The Black Riders do not scream, but keep a more terrifying silence. Aragorn does not blanch. The riders draw slowly in on foot in darkness, and do not `spur'. There is no fight. Sam does not `sink his blade into the Ringwraith's thigh', nor does his thrust save Frodo's life. (If he had, the result would have been much the same as in III 117-20: the Wraith would have fallen down and the sword would have been destroyed)." [Letter #210 FROM Tolkien, dated 1958 to a Mr. Forrest J. Ackerman]|
Sam's blade is like Merry's blade - coming from the Barrow-downs and having been made by the Men of Westernesse with spells of ruin for the Witch-king, HOWEVER - Tolkien states that if Sam had stabbed the Nazgul he would have "fallen down" - this is a far cry from killing, vanquishing, mortally wounding, or whatever term you want to use.
In this same letter there is a footnote for further clarification: Footnote #4: "The slaying of the Lord of the Nazgul by Eowyn."
And there is the entry in the Index to "Unfinished Tales" as compiled by Tolkien's son, Christopher, who has been the editor of his father's notes and certainly knew the mind of his father's works and intents more than any others:
|"Eowyn Sister of Eomer, wife of Faramir; slayer of the Lord of the Nazgul"|
Certainly and unequivocally, Merry's blow was crucial - it allowed that precious moment of time for Eowyn to rise up and kill the Witch-king, so it was a team effort!
So, Merry gets the assist and Eowyn the point.