Why is the word "Vayikachuhu" missing a 'Vav' (after the 'Ches)?
Ba'al ha'Turim: Because it was only Shimon who took him and threw him into the pit. 1
Having said that the pit was empty, why does the Torah add that it contained no water?
Ramban #1 (on Pasuk 22) and Rashbam: The Torah tells us that the pit was not full of water, because if it had, having decided not to kill him directly, they would not have thrown him into it.
Ramban #2: To teach us that not only was the pit an empty one, but there was not even a small amount of water in it. 4
Divrei Eliyahu: Mayim alludes to Torah. One who has no water (Torah) is prone to have snakes and scorpions, i.e. despicable Midos like snakes and scorpions. 2
Da'as Zekenim, Hadar Zekenim: Two exclusions always come to include. Do not say that it contained rocks, for 'Rak" connotes something that sometimes it is Misroken (emptied) from it. Hadar Zekenim - i.e. it hides in cracks. Alternatively, we read "Yashvu Le'echol Lechem" like 'Yesh Bo Le'echol Lechem' (there is something inside that can eat; Rosh - there is something edible inside). Moshav Zekenim - or, we learn a Gezeirah Shavah "Mayim-Mayim" - "Nachash Saraf v'Akrav v'Tzima'on Asher Ein Mayim" (Devarim 8:15). 3
See also Ba'al ha'Turim. The brothers however, were unaware of this (either due to the depth of the pit or because the snakes were hidden in the crevices in the side of the pit). Had they known , they would have realized, when Yosef emerged alive from the pit, that he was a great Tzadik, and they would never have gone on to sell him into slavery (See Daniel, 6:23 [Ramban]). In any event, based on their decision not to kill him directly, they would not have thrown him into a pit with snakes and scorpions, like the Ramban says (refer to 37:24:2:2). Moshav Zekenim, Riva (22) - surely Reuven did not know about the snakes and scorpions, for the Torah says that he intended to save him! Alternatively, he knew; even so, this is better than overtly killing him, for perhaps Yosef is a snakecharmer and can save himself (Riva). Moshav Zekenim - the pit was wide, so Yosef would not need to fall on the snakes, so they would not necessarily kill him.
רש"י: אבל נחשים ועקרבים יש בו: לכאורה היה אפשר לדרוש באותה הדרך שעצים ואבנים יש בו?
גור אריה: אם היו בו עצים ואבנים לא הייתה התורה אומרת "והבור ריק", אבל נחשים ועקרבים אינם נחשבים לממלאים את הבור כי הם נמצאים כאן לפי שעה והולכים.