All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Saying that some scholars are of the view that if someone kills more than three people he has to fast for 180 days (i.e. for three people only), is not correct as we never heard anyone of them [scholars] saying this, and no books of Fiqh (Islamic jurisprudence), despite their great number, mentioned this. So we do not know from where you brought this statement. The evidence from the Book of Allaah, the Sunnah of the Prophet and the books of jurists is that whoever kills a group of people by mistake, he has to expiate for each person that he killed as each person is a separate case. The expiation is to free a Muslim slave, and if one is unable to do so, he has to fast two consecutive months; there is an evidence about this in the book of Allaah. Allaah says (which means): {And whosoever kills a believer by mistake, (it is ordained that) he must set free a believing slave and compensation (blood money, i.e. Diyah) be given to the deceased's family, unless they remit it. If the deceased belonged to a people at war with you and he was a believer; the freeing of a believing slave (is prescribed), and if he belonged to a people with whom you have a treaty of mutual alliance, compensation (blood money-Diyah) must be paid to his family, and a believing slave must be freed. And whosoever finds this (the penance of freeing a slave) beyond his means, he must fast for two consecutive months in order to seek repentance from Allaah. And Allaah is Ever All-Knowing, All-Wise.}[4:92]. However, if one is unable to fast, he has to feed sixty poor people for each soul, and this is by analogy; this is the view of the Shaafi'ee school and a narration by Ahmad.
However, one could say that a person is unable to fast because of the length of time, like in the case of a person who kills 50 people in an accident for instance, [in which case he has to fast for nearly nine years]. If we take off the month of Ramadan from each year and the days of 'Eed, and the days of sickness, then the period will be about ten years or more, and he does not break the fast except in days of 'Eed, and this entails great hardship and difficulty, and hardship necessitates easiness (an Islamic fundamental rule). So in such a case it could be said that if he is able to expiate (fast) for some people, then he has to do so without restricting the number to three people or less or more. And he has to feed the rest if he is able to do so. Allaah says (which means): {So keep your duty to Allaah and fear Him as much as you can.}[64:16]. This is in accordance to the jurists who say that one has to feed as the expiation for killing a person when he is unable to do the other things.
Allaah knows best.