I discussed the following interesting Minchas Chinuch with my son over Shabbos: The halacha is that we pasken dinei nefashos based on rov (Sanhedrin 69). We also know that rov gosisin l’misah. It should therefore follow that if Reuvain makes Shimon into a goseis (and Shimon is subsequently killed by Levi, so we can’t just wait and see if he recovers or not), Reuvain would be chayav for murder. By way of analogy: if Reuvain pushed Shimon off a cliff and on the way down Shimon was shot by Levi so we don't know if he would have miraculously survived, Reuvain is guilty of murder (thanks R’ P.C. for the analogy).
The logic sounds right, but Tosfos (Sanhedrin 78a d”h b’goseis) writes that Reuvain is not chayav for making Shimon a goseis. Putting aside Tos’ proofs (which the Minchas Chinuch takes issue with), why does the principle of rov not apply in this case?