From a human standpoint – certainly not. But Eru wasn't human, and saw the world with different eyes. I don’t want to advocate the death of so many people of course; this answer is merely an attempt to explain a part of the Númenor legend without using the valid but boring Eru-is-an-a**hole-explanation.
Preventing a conflict
First, there are the immediate consequences if he had not intervened at all. This would have forced the Valar to confront the Númenoreans, Eru's children, directly with force – something they had not done before in this way. Maybe they would have refused, because they had come to Arda to prepare and guard the world for the Children, and not fight them. In this case, a confrontation between the Númenoreans and the Eldar of Aman would have been inevitable, a conflict between these two kindred of a scale that had not been before either. It would have had horrible results for both sides.
So by intervening, Eru protected not only the Eldar from the Númenorean invasion, but the Valar as well – who were, after all, created by him as well.
The Shadow on Númenor
So most people can probably understand why something had to be done about the armada. But what about Númenor itself? What about all the people on the island who were not part of the attack on Valinor?
I think, the main reason for that was actually Sauron. After all, not being part of the armada did not make the people on Númenor innocent after all. Sauron was the prime example – he stayed on the island, but was actually the major culprit in everything the Númenoreans had done. We can argue about the lack of involvement of the Valar in Middle-earth after the War of Wrath, but in this case fighting against Sauron would have meant fighting against Númenor. Obviously the Valar did not want to do this, and when they called on Eru, he not only destroyed the armada but also dealt a blow against Sauron. When the Valar fought their war against Morgoth, a whole continent was drowned. It’s not exactly surprising that Eru’s strike against Sauron sunk an island.
But Sauron was probably not the only aim on Númenor. We should not forget that all the inhumane things that the Númenoreans had done were supported by a large part of the population. Slavery and human sacrifices had become acceptable in their society, and most of Númenor had fallen under Sauron’s shadow.
“Thereafter the fire and smoke went up without ceasing; for the power of Sauron daily increased, and in that temple, with spilling of blood and torment and great wickedness, men made sacrifice to Melkor that he should release them from Death. And most often from among the Faithful they chose their victims; yet never openly on the charge that they would not worship Melkor, the Giver of Freedom, rather was cause sought against them that they hated the King and were his rebels, or that they plotted against their kin, devising lies and poisons.
[…]
And they sailed now with power and armoury to Middle-earth, and they came no longer as bringers of gifts, nor even as rulers, but as fierce men of war. And they hunted the men of Middle-earth and took their goods and enslaved them, and many they slew cruelly upon their altars.”¹
However, those that had not fallen under the shadow yet and still opposed Sauron got a chance to escape. The Faithful were saved “by grace of the Valar” and sailed to Middle-earth, evading the Fall of Númenor.
Now often at this point Tar-Míriel is mentioned. What about the unhappy Queen of Númenor, who was according to the Akallabêth a victim of Ar-Pharazôn as well?
As an out-of-universe-answer I’d say that her story was still work in progress. Tolkien tried several versions of her story, and it’s not completely clear what his last thoughts on the matter were. In some versions of this story Tar-Míriel is in love with Ar-Pharazôn and breaks off a previous engagement to marry him. One of these version reads:
“But Pharazôn [?arose] and came to her, and she was glad, and forsook the allegiance of her father for the time, being enamoured of Pharazôn. And in this they broke the laws of Númenor that forbade marriage even in the royal house between those more nearly akin than cousins in the second degree. But they were too powerful for any to gainsay them. And when they were wedded she yielded the sceptre to Pharazôn, and he sat upon the throne of Elros in the name of Ar-Pharazôn the Golden, but she retained also her title as hers by right, and was called Ar-Zimrahil.”²
The Míriel in this version surely would not have been as innocent in all of Ar-Pharazôn’s doings as the Míriel in the published version.
For the in-universe-answer, let’s talk about the fate of Men in Arda.
Guests in Arda
”It is one with this gift of freedom that the children of Men dwell only a short space in the world alive, and are not bound to it, and depart soon whither the Elves know not. […] But the sons of Men die indeed, and leave the world; wherefore they are called the Guests, or the Strangers. Death is their fate, the gift of Ilúvatar, which as Time wears even the Powers shall envy.”³
Men were not meant to live in Arda Marred for a long time. They had short lives and died soon, leaving Arda, though nobody among the Children of Eru knew where they would go. Men did not see this as something positive, but if it indeed was meant as a gift by Eru, and the fear of death only the result of Melkor’s shadow, then it makes sense that Eru would not see the death of Men as a necessarily bad thing. They were, after all, made to “seek beyond the world and should find no rest therein”³.
If Men used what little time they had to further a development that obviously went against Eru’s plans for Arda, I don’t think he would see anything ill in ending the lives of the Númenoreans sooner, leaving only those alive that were still on the “right track”.
”The Eagles of Manwë” by Ted Nasmith
Free will exists in Middle-earth, even though it’s not absolute. According to The Silmarillion, Men especially “should have a virtue to shape their life, amid the powers and chances of the world, beyond the Music of the Ainur, which is as fate to all things else”³. Therefore, listening to Sauron was a decision made by the Númenoreans and probably would have had even more disastrous consequences than the Fall of Númenor if Eru had not intervened. The Fall had not been part of the original plan for Arda, but as Men made choices that would affect their own lives and the world around them, Eru made choices for Arda as a whole, resulting not only in the loss of Númenor but also in the loss of Aman as a part of the rest of the world.
“[…] one of the Eldar would have said that for all Elves and Men the shape, condition, and therefore the past and future physical development and destiny of this ‘earth’ was determined and beyond their power to change, indeed beyond the power even of the Valar to alter in any large and permanent way. […] The Downfall of Númenor was ‘a miracle’ as we might say, or as they a direct action of Eru within time that altered the previous scheme for all remaining time.”⁴
So even if from a human perspective the drowning of Númenor seems too cruel and merciless as a reaction, we have to remember that Eru did not see death as something evil at all. The corruption of the Númenoreans by Sauron might have even seen like the greater evil to him.
Tolkien’s dream
There is a reason outside of the Legendarium for the Fall of Númenor: since childhood Tolkien had had repeated dreams of it, and that’s why it comes up in many parts of his writings, even if it’s only little bits and pieces.
There is of course the Akallabêth and its predecessors The Fall of Númenor and The Drowning of Anadûnê. Then there is The Lost Road, where images of the Fall of Númenor and the Elvish language come to Alboin in dreams. There are The Notion Club Papers, where both Ramer and Lowdham dream of Númenor and its fall. And in The Lord of the Rings, Faramir dreams of Númenor’s end.
In one of his letters Tolkien writes:
“I say this about the 'heart', for I have what some might call an Atlantis complex. Possibly inherited, though my parents died too young for me to know such things about them, and too young to transfer such things by words. Inherited from me (I suppose) by one only of my children, though I did not know that about my son until recently, and he did not know it about me. I mean the terrible recurrent dream (beginning with memory) of the Great Wave, towering up, and coming in ineluctably over the trees and green fields. (I bequeathed it to Faramir.) I don't think I have had it since I wrote the 'Downfall of Númenor' as the last of the legends of the First and Second Age.”⁵
So in a way, Númenor was always fated to sink, because its creator had visions of it that would not stop until he brought it to life on paper.
Who knows – maybe Eru felt the same.
Thanks for the A2A!
FOOTNOTES
¹ J.R.R. Tolkien, Christopher Tolkien. The Silmarillion, Akallabêth: The Downfall of Númenor.
² J.R.R. Tolkien, Christopher Tolkien. The History of Middle-earth XII: The Peoples of Middle-earth, 1, V: The History of the Akallabêth, Note on the marriage of Miriel and Pharazon, (a)
³ J.R.R. Tolkien, Christopher Tolkien. The Silmarillion,Quenta Silmarillion, Chapter 1.
⁴ J.R.R. Tolkien, Carl F. Hostetter. Fate and Free Will (Tolkien Studies: Volume 6).
⁵ J.R.R. Tolkien, Humphrey Carpenter. The Letters of J.R.R. Tolkien, Letter #163.