Why does the Torah write "v'Lo Yishmerenu" by Mu'ad and not by Tam?
Ramban #1: According to the opinion that a Mu'ad requires a better guarding than a Tam, 1 it teaches us that the minimal Shemirah that will suffice for a Tam will not suffice for a Mu'ad.
Ramban #2: According to those who hold that both require only a minimal Shemirah, it is informing us that, since, even after it became a Mu'ad, the owner still failed to guard it, he is obligated to pay full damages.
See Rav Chavel's footnotes.
What are the connotations of "veha'Meis Yihyeh lo"?
Ramban #2: The Neveilah belongs to the Mazik, to include in the payment to the Nizak. 3
See also Ba'al ha'Turim.
Ramban: In which case, what the Pasuk is saying is that the Mazik pays full damage, including the Neveilah [which he returns to the victim]. The 'Vov' means 'with' - similar to Shemos 1:5 and Yeshayah, 1:13.
Rashi writes that "veha'Mes Yihyeh Lo" teaches that the Neveilah belongs to the Nizak and the Mazik pays the balance. Bava Kama 51a expounds this to exclude Pesulei ha'Mukdashim!