Three thoughts on this daf:
1)
אפילו תימא ר' יאשיה לחלק הכא לא צריך מ"ט סברא הוא מה לי קטלה כולה מה לי קטלה פלגא
What is this half-killing and whole killing? The standard explanation is as Rashi writes.
סברא הוא - דנשבר בלא מת נמי מחייב דהוה ליה קטלה פלגא:
That is, that a break in the animal, which is considered half-killed, he should also be liable for, for half-killed is like whole-killed.
My issue with this is that a broken leg is not half-killed. It is damaged. It is so obvious that there should be no distinction between (what we are really saying) full and half damage?
I would suggest that whole killed is the broken + died. That animal is really really dead. And half-killed is where it died without first sustaining an injury which broke it. The sevara is then this: are you truly going to say that it needs to be both broken and dead for him to pay, but simply dead with no break, he would be exempt? That would be ridiculous. And once we have dead alone for liability, if broken is mentioned, it is also a separate case for liability. Therefore, we don't need an או to divide, since even with a ו, we would know that we are dealing with separate cases.
2) The sometimes forced-seeming derashot aside, how can we read the pesukim on a peshat-like level to arrive at this conclusion, that even by other shomrim, if the owner is hired/borrowed with the onset of the animal, he is exempt?
The two pesukim were:
וְכִי יִשְׁאַל אִישׁ מֵעִם רֵעֵהוּ וְנִשְׁבַּר אוֹ מֵת בְּעָלָיו אֵין עִמּוֹ שַׁלֵּם יְשַׁלֵּם.
אִם בְּעָלָיו עִמּוֹ לֹא יְשַׁלֵּם אִם שָׂכִיר הוּא בָּא בִּשְׂכָרוֹ.
The first pasuk speaks of the borrower, and comes to teach that unlike the watchmen of previous pesukim, he is liable for ones. So long as there is not this overriding be'alav imo, which would exempt him.
The second pasuk does not explicitly say he is a borrower. It lays out an exemption if the owner is there with the animal. So the question is how far this exemption of be'alav imo applies. Is it a mere exemption to the higher liability for ones? Or is it a trump card for everything, which was finally brought in here, because it is the (generally applying) exemption exception that sustains even now that we have made him liable for ones.
3)
ת"ש שאלה ושאל בעליה עמה שכרה ושכר בעליה עמה שאלה ושכר בעליה עמה שכרה ושאל בעליה עמה אף על פי שהבעלים עושין מלאכה במקום אחר ומתה פטור
Renting the animal / owner isn't just an random example. I think there is a hidden derasha operating, about אִם שָׂכִיר הוּא בָּא בִּשְׂכָרוֹ
1)
אפילו תימא ר' יאשיה לחלק הכא לא צריך מ"ט סברא הוא מה לי קטלה כולה מה לי קטלה פלגא
What is this half-killing and whole killing? The standard explanation is as Rashi writes.
סברא הוא - דנשבר בלא מת נמי מחייב דהוה ליה קטלה פלגא:
That is, that a break in the animal, which is considered half-killed, he should also be liable for, for half-killed is like whole-killed.
My issue with this is that a broken leg is not half-killed. It is damaged. It is so obvious that there should be no distinction between (what we are really saying) full and half damage?
I would suggest that whole killed is the broken + died. That animal is really really dead. And half-killed is where it died without first sustaining an injury which broke it. The sevara is then this: are you truly going to say that it needs to be both broken and dead for him to pay, but simply dead with no break, he would be exempt? That would be ridiculous. And once we have dead alone for liability, if broken is mentioned, it is also a separate case for liability. Therefore, we don't need an או to divide, since even with a ו, we would know that we are dealing with separate cases.
2) The sometimes forced-seeming derashot aside, how can we read the pesukim on a peshat-like level to arrive at this conclusion, that even by other shomrim, if the owner is hired/borrowed with the onset of the animal, he is exempt?
The two pesukim were:
וְכִי יִשְׁאַל אִישׁ מֵעִם רֵעֵהוּ וְנִשְׁבַּר אוֹ מֵת בְּעָלָיו אֵין עִמּוֹ שַׁלֵּם יְשַׁלֵּם.
אִם בְּעָלָיו עִמּוֹ לֹא יְשַׁלֵּם אִם שָׂכִיר הוּא בָּא בִּשְׂכָרוֹ.
The first pasuk speaks of the borrower, and comes to teach that unlike the watchmen of previous pesukim, he is liable for ones. So long as there is not this overriding be'alav imo, which would exempt him.
The second pasuk does not explicitly say he is a borrower. It lays out an exemption if the owner is there with the animal. So the question is how far this exemption of be'alav imo applies. Is it a mere exemption to the higher liability for ones? Or is it a trump card for everything, which was finally brought in here, because it is the (generally applying) exemption exception that sustains even now that we have made him liable for ones.
3)
ת"ש שאלה ושאל בעליה עמה שכרה ושכר בעליה עמה שאלה ושכר בעליה עמה שכרה ושאל בעליה עמה אף על פי שהבעלים עושין מלאכה במקום אחר ומתה פטור
Renting the animal / owner isn't just an random example. I think there is a hidden derasha operating, about אִם שָׂכִיר הוּא בָּא בִּשְׂכָרוֹ