Is a person's sacrificing of himself for another and dying in his stead in an attempt to save him considered suicide or unlawful, even if there is no kinship between them?

A person sacrificing himself for another is not suicide; rather, it is a high degree of altruism. This occurred in the Battle of Yarmouk, where three Companions passed water among themselves, each preferring his companion over himself, until they died without drinking. However, if sacrificing oneself leads to an action by which the person causes his own death, such as donating vital organs upon which his life depends, then this is considered suicide. And the one who commits suicide is threatened with eternity in the fire of Hell.