In canon they believe this is the requirement. I argue that this makes no sense. It is inconceivable to me that over the course of however many hundreds or thousands of years the Uchiha Clan existed, absolutely zero Uchiha have seen their close friend die without having killed them, gained the Mangekyo, and therefore invalidated the previous assumption. There are simply too many plausible outcomes: KIA, suicide, training (or otherwise) accident, poison, disease. You could argue that the Uchiha Clan would assume that the new Mangekyo-wielders actually killed said friend, but then the Uchiha Clan would have way way way too many kinslaying Treason Dragons on their hands to justify; they'd need to re-evaluate their assumptions or go extinct killing themselves off.