Why is Moshe's death after the war against Midian?

Summary: Last year, I presented one reason from Rav Yonasan Eibeshitz. Here is another one, having to do with the laws of ritual purity.

Post: The pasuk in Matos, with the instruction to take revenge on the Midianites, is:

ב  נְקֹם, נִקְמַת בְּנֵי יִשְׂרָאֵל, מֵאֵת, הַמִּדְיָנִים; אַחַר, תֵּאָסֵף אֶל-עַמֶּיךָ.

2 'Avenge the children of Israel of the Midianites; afterward shalt thou be gathered unto thy people.'

And there are various midrashim about this juxtaposition. For instance, that Moshe immediately acted, without delay, even though this meant his demise. But other diyukim are made into just why this juxtaposition is made. Here is another one by Rav Yonasan Eibeshitz. In Midrash Yehonasan, he writes:

That is, he refers us to Sanhedrin daf 39a:

A certain Min said to R. Abbahu: Your God is a priest, since it is written, That they take for me Terumah [wave offering].³⁹  Now, when He had buried Moses,⁴⁰  wherein did He bathe [after contact with the corpse]?⁴¹  Should you reply, 'In water: is it not written, Who hath measured the waters in the hollow of His hand?⁴²  — 'He bathed in fire,' he answered, 'for it is written, Behold the Lord will come in fire.'⁴³  'Is then purification by fire effective?' 'On the contrary,' he replied, 'bathing [for purposes of purification] should essentially be in fire, for it is written, And all that abideth not the fire ye shall make to go through the water.'⁴⁴ 

This is a nice back-and-forth, with perhaps a hidden encoded meaning. And this may require an understanding of Christian theology. I don't possess such knowledge, but I can Google. Thus, for instance:

The best place to start any discussion of this ritual is Erdsman's Le Baptême de Feu. Here Origen is cited as having consistently confirmed the baptism of fire as a purification ritual:

At the Jordan river, John awaited those who came for baptism. Some he rejected, saying, "generation of vipers," and so on.3 But those who confessed their faults and sins he received. In the same way, the Lord Jesus Christ will stand in the river of fire near the "flaming sword." If anyone desires to pass over to paradise after departing this life, and needs cleansing, Christ will baptize him in this river and send him across to the place he longs for. But whoever does not have the sign of earlier baptisms, him Christ will not baptize in the fiery bath. For, it is fitting that one should be baptized first in “water and the Spirit.” Then, when he comes to the fiery river, he can show that he preserved the bathing in water and the Spirit. Then he will deserve to receive in addition the baptism in Christ Jesus, to whom is glory and power for ages of ages. Amen

Thus, in this gemara, the Min, which means early Christian, believed in the necessity of baptism, purification by water, and was told that actually the purification by fire is the preferable one. Perhaps; perhaps not. Perhaps it has to do with the limitations on the Infinite, such that the Min was arguing that it could not have been Hashem Himself who buried Moshe. See here for an example of a Christian who believes that Christ buried Moses, rather than Hashem. Similarly, the prooftext that Hashem is a kohen also could be a reference to Hashem restricting Himself, so as to dwell amongst the bnei Yisrael {vayikchu li terumah veshachanti besocham}.

(If I wanted to reject the Min's words, I would point out that Hashem is Himself compared to fire. Thus, on the pasuk in Devarim,

Devarim 10:20

And (1) you will fear the Lord, your God; (2) Him you shall worship/serve; and (3) to Him you will cleave; and (4) by His Name you shall swear.

Ketubot 111b asks,

…Is it possible to cleave to the Divine Presence? Isn’t it written, (Devarim 4:24) “Because the Lord, your God, is a Consuming Fire, He is a Zealous God”?  But rather anyone who marries off is daughter to a Tora scholar, who engages in business arrangements with Tora scholars, who benefits Tora scholars from his possessions, the text considers him as if he has cloven to the Holy One, Blessed Be He…

And we know fire is not susceptible to ritual impurity. Thus, Berachot 22a:

"It has been taught, Rabbi Judah ben Bathyra used to say: The words of the Torah are not susceptible to Tumah [commonly translated 'impurity']. It happened that a disciple standing before Rabbi Judah ben Bathyra spoke hesitatingly [evidently he was tameh (one who is in a state of Tumah)- being a ba'al keri, one who has had a seminal emission- thought that he was debarred from uttering words of Torah]. He said to him: My son, open your mouth and let your words be clear, for the words of the Torah are not susceptible to Tumah. For it is said: "Is not My word like fire, says the Lord" (Jeremiah 23:29)? As fire is not susceptible to Tumah, so are words of Torah not susceptible to Tumah."

And so Hashem, although being a kohen, would not be susceptible to tumah.)

Rav Yonasan Eibeshitz, for his purposes, takes this gemara literally. And since the pasuk about purification of vessels by fire is stated about the vessels captured in Midian, until that law was given, Hashem was not about to bury Moshe, and so Moshe was not ready to die.