It is only the Watchtower that focuses on Jesus' death as a 'ransom'. Russell taught that Jesus died to redeem Adam who would be saved. We were all saved - or given a second chance - by proxy. Rutherford cancelled Adam's redemption and changed it into the value of the perfect life that Adam forfeited.
Ransom is only one of a number of metaphors that NT writers used to describe the effect of Jesus' death. It was a reference to the idea of the emancipation of a slave. Like most metaphors they make less sense the more literally you try to apply them. Primarily early christians - and modern ones - see Jesus' death in terms of vicarious punishment. In other words his punishment on the cross was the punishment christians deserve for their sins. It was this concept that Nelson Barbour rejected leading to a split with Russell.
Why did Jesus die?
JW - to buy back the perfect life that Adam lost by his disobedience.
Christian - He took my punishment for my sins. 'By his stripes we were healed'.