How did Reuven save Yosef by casting him into a pit filled with Snakes and Scorpions?

Summary: If there was no water, but instead snakes and scorpions, then how was Reuven saving him by casting him into the pit?

Post: In parashat Vayeshev, in order to save Yosef from being directly murdered by his brothers, Reuven suggests casting Yosef into a pit. Thus, the pesukim with Rashi:

21. But Reuben heard, and he saved him from their hand[s], and he said, "Let us not deal him a deadly blow."		כא. וַיִּשְׁמַע רְאוּבֵן וַיַּצִּלֵהוּ מִיָּדָם וַיֹּאמֶר לֹא נַכֶּנּוּ נָפֶשׁ:
“Let us not deal him a deadly blow.”: Heb. לֹא נַכֶּנוּ נָפֶשׁ. Literally, let us not smite him the soul. [This is equivalent to] מַכַּת נֶפֶשׁ, [let us not deal him] a deadly blow, which means death. [From Targum Onkelos]		לא נכנו נפש: מכת נפש זו היא מיתה:
22. And Reuben said to them, "Do not shed blood! Cast him into this pit, which is in the desert, but do not lay a hand upon him," in order to save him from their hand[s], to return him to his father.		כב. וַיֹּאמֶר אֲלֵהֶם רְאוּבֵן אַל תִּשְׁפְּכוּ דָם הַשְׁלִיכוּ אֹתוֹ אֶל הַבּוֹר הַזֶּה אֲשֶׁר בַּמִּדְבָּר וְיָד אַל תִּשְׁלְחוּ בוֹ לְמַעַן הַצִּיל אֹתוֹ מִיָּדָם לַהֲשִׁיבוֹ אֶל אָבִיו:
to save him: The Holy Spirit testifies for Reuben that he said this only to save him, so that he would [be able to] come and take him out of there. He said, “I am the firstborn and the eldest of them all. The sin will be attributed only to me.” [from Gen. Rabbah 84:15]		למען הציל אותו: רוח הקודש מעידה על ראובן שלא אמר זאת אלא להציל אותו, שיבא הוא ויעלנו משם, אמר אני בכור וגדול שבכולן, לא יתלה הסרחון אלא בי:

Yet, the pit was a dangerous pit! A few pesukim later:

24. And they took him and cast him into the pit; now the pit was empty there was no water in it.		כד. וַיִּקָּחֻהוּ וַיַּשְׁלִכוּ אֹתוֹ הַבֹּרָה וְהַבּוֹר רֵק אֵין בּוֹ מָיִם:
now the pit was empty-there was no water in it: Since it says: “now the pit was empty,” do I not know that there was no water in it? For what purpose did the Torah write,“there was no water in it” ? [To inform us that] there was no water in it, but there were snakes and scorpions in it. [From Shab. 22a, Chag. 3a]		והבור רק אין בו מים: ממשמע שנאמר והבור רק, איני יודע שאין בו מים, מה תלמוד לומר אין בו מים, מים אין בו אבל נחשים ועקרבים יש בו:

How in the world is this "saving" Yosef?

Chizkuni asks this question:

If his intent was to save him from their hands, and to return Yosef to his father, then why cast him into a pit full of snakes and scorpions? Chizkuni suggests that there were actually two pits. The one Reuven suggested with הַבּוֹר הַזֶּה didn't have snakes and scorpions, but once Reuven left, they cast him into a different pit.

This might seem somewhat farfetched, but on the other hand, I could point to some language in the midrash which might in fact indicate this:

והבור רק אין בו מים מים אין בו אבל נחשים ועקרבים יש בו.
שני בורות היו: אחד מלא צרורות.
ואחד מלא שרפים ועקרבים.

אמר רבי אחא:בור רק, נתרוקן בורו של יעקב.
אין בו מים, אין בו דברי תורה, שנמשלו למים, היך מה דאת אמר (ישעיה נה) הוי כל צמא לכו למים. 
כתיב: (דברים כד) כי ימצא איש גונב נפש מאחיו, ואתם מוכרים את אחיכם?!

Thus, there were two pits, one filled with pebbles and the other filled with snakes and scorpions. While the derasha might be from the extra word bor, as various meforshei midrash suggest. But textual issues aside, I think it might be prompted by the desire to solve the narrative issue of just how Reuven was trying to save Yosef.

I would suggest something simpler. Either that when Reuven suggested this, he did not look into the pit, and by the time they lowered Yosef into the pit, Reuven had already left; or, even the brothers were unaware that there were snakes and scorpions in the pit, and the intent of the midrash is to show even further Divine protection.

Gur Aryeh asks a related question. He contrasts Reuven's intent to save Yosef with the gemara in Yevamot 121a which says anyone cast into a pit filled with snakes and scorpions will surely die.

He answers that the gemara in Yevamot is dealing with a pit filled to the brim with snakes and scorpions, while the midrash only speaks of a pit in which snakes and scorpions yesh bo. Meanwhile, the brothers believed that this would kill Yosef indirectly (and thus Yehuda later speaks of not killing Yosef), but Reuven throughout thought that the snakes would likely not kill Yosef immediately, and so he would have time to go back and save him. It works out nicely, but it involves adding a lot of details and motivations which are not clearly spoken out in any midrash, at each stage. This is what often causes me to regard Gur Aryeh's explanations as neo-midrash. In this case, many of these appear as logical consequences of previous assessments, and assertions of Rashi/midrash. So perhaps.

The gemara, in Yevamot 121a, reads:

תניא א"ר עקיבא פעם אחת הייתי מהלך בספינה וראיתי ספינה אחת שמטרפת בים והייתי מצטער על תלמיד חכם שבה ומנו רבי מאיר כשעליתי למדינת קפוטקיא בא וישב ודן לפני בהלכה אמרתי לו בני מי העלך אמר לי גל טרדני לחברו וחברו לחברו עד שהקיאני ליבשה אמרתי באותה שעה כמה גדולים דברי חכמים שאמרו מים שיש להם סוף אשתו מותרת מים שאין להם סוף אשתו אסורה תנו רבנן נפל לגוב אריות אין מעידין עליו לחפורה מלאה נחשים ועקרבים מעידין עליו רבי יהודה בן בתירא אומר אף לחפורה מלאה נחשים ועקרבים אין מעידין עליו חיישינן

שמא חבר הוא ות"ק אגב איצצא מזקי ליה 

It was taught: R. Akiba related, 'l was once travelling on board a ship when I observed a ship in distress,⁴⁶  and was much grieved on account of a scholar who was on it. (And who was it? — R. Meir.) When I subsequently landed in the province of Cappadocia⁴⁷  he came to me and sat down and discussed matters of halachah. "My son", I said to him, "who rescued you?" — "One wave" he answered me, "tossed me to another, and the other to yet another until [the sea] cast me⁴⁸  on the dry land". At that hour I exclaimed: How significant are the words of the Sages who ruled [that if a man fell into] water which has [a visible] end, [his wife] is permitted [to marry again; but if into] water which has no [visible] end, she is forbidden'. Our Rabbis taught: If a man fell into a lion's den, no evidence⁴⁹  may be legally tendered concerning him;⁵⁰  but if into a pit full of serpents and scorpions, evidence⁴⁹  may legally be tendered concerning him.⁵⁰  R. Judah b. Bathyra ruled: Even [if he fell] into a pit full of serpents and scorpions, no evidence⁴⁹  may legally be tendered concerning him,⁵⁰  since the possibility must be taken into consideration

that he might be a charmer.¹  But the first Tanna?²  — Owing to the pressure³  they⁴  injure him.⁵ 

I am not sure whether the diyuk in the word מלאה  is spot on. That word need not mean "filled to the brim", but can merely mean containing. On the other hand, the gemara does explain the position of the Tanna Kamma that אגב איצצא מזקי ליה, and there would certainly be greater pressure if it was entirely filled with snakes and scorpions. Back to the first hand, the point may be that it is a more confined space, such that he would end up touching or pressing against one, not that there are snakes and scorpions over every inch of his body, and then some.

I would add something more than this to the above paragraph. Recall that Gur Aryeh tries to make the distinction between the gemara, where the pit is malei with snakes and scorpions, and the pit mentioned by Rashi, which they are merely yesh bo. However -- and here we see the importance of looking up the sources in the original -- in Midrash Rabba, which precedes Rashi in stating that there were snakes and scorpions, it is stated as:

והבור רק אין בו מים מים אין בו אבל נחשים ועקרבים יש בו.
שני בורות היו: אחד מלא צרורות.
ואחד מלא שרפים ועקרבים.

Note the phrase מלא שרפים ועקרבים. The diyuk doesn't work if both sources declare the pits as malei with snakes and scorpions!
Here is how the Taz deals with it:

That is, the snakes and scorpions were in cracks and crevices, such that Reuven did not see them there. For otherwise, how in the world would he be saving Yosef?!

I would note the alternate derasha by Rabbi Acha in Midrash Rabba, that bor rek means that the pit / descendants of Yaakov were empty, and ain bo mayim means that there was no Torah in it, for Torah is compared to water.

Several years ago, my father suggested a similar peshat based only on the portion which is cited by Rashi. The bor was empty, such that there was nothing in it. Ain bo mayim. And mayim always refers to Torah. One would think that if something is empty of Torah, it is simply empty, in a neutral state. But no. תלמוד לומר אין בו מים, מים אין בו אבל נחשים ועקרבים יש בו. If one is empty of Torah, all sorts of evil influences enter in its place.