The Gemora (Bava Kamma 4a) explained the Mishna according to Rav as follows: The nature of ox, that it pays kofer (if the ox kills a person, the owner is liable to pay a kofer payment), is not the same as the nature of a person, who does not pay kofer (and therefore, if the Torah would only write that one is liable for an ox, we would not necessarily know that one is liable for his own damaging).
Tosfos asks: Why is the fact that an ox pays kofer regarded as a stringency? The reason why a person does not pay kofer when he kills someone is because of the principle of kim leih bid’rabbah minei - (whenever someone is deserving of two punishments, he receives the one which is more severe). And since a man is executed for killing another man, he is not required to pay the kofer payment. It emerges that not paying the kofer is not a leniency, but rather, it is due to a stringency, namely – that he is put to death!?
The Riva answers that a person would not pay kofer even if he would not be executed. This would be in a case of an accidental killing.
The Darchei Dovid explains: Although this is also a type of kim leih bid’rabbah minei; whenever someone accidentally does something - and this same action, if it would have been done intentionally, would have exempted him from a monetary payment – it exempts him from the payment in this case as well; nevertheless, it must be regarded as a lenient ruling, for the bottom line is that he is not executed and he is not obligated to pay anything.
Alternatively, Tosfos answers that he is not exempt from the kofer payment due to kim leih bid’rabbah minei.
The Maharam explains: The principle of kim leih bid’rabbah minei only applies when he committed an action that he deserves to be executed for, and simultaneously, he does something else that he is required to pay money for. However, in our case, where a man killed someone, the punishment of execution and the kofer payment are both coming for the same reason. Kim leih bid’rabbah minei will not apply here.
The Reshash explains Tosfos to mean that the principle of kim leih bid’rabbah minei does not apply in this case because the kofer payment serves as an atonement for the killing. Kim leih bid’rabbah minei exempts a money obligation which is a payment because of compensation; however, it does not exempt payments on account of forgiveness.