Jesus Christ is known to have saved us - but what from?
From what the Ancient Hebrews of the Old Testament called Sheol (which is much the same a what the Ancient Greeks called Hades). And the descriptions of Sheol/ Hades are of a state of discarnate un-consciousness.
So - before Christ, when mortal Men died, they lost their bodies and their 'wits' - they become 'demented', lost self-awareness, lost the meaning and purpose of existence, lost all relationships with other Men and with God.
And this is what Christ saved us from - because after Christ Men are resurrected - that is they have restored and perfected bodies, and become again capable (indeed in an enhanced way) of consciousness: they have real relationships, their lives are self-aware with meaning and purpose.
Sheol/ Hades is the consequence of the death of an incarnate mortal. Yet (Mormons believe, and so do I) that before mortality we had a spiritual existence in which we were conscious souls, had relationships, meaning and purpose.
From this I infer that our incarnation as mortal Men was not merely our souls being given a body - but that the soul and body become one, interpenetrated - such that when the body dies, our souls are left incomplete - as described for Sheol/ Hades.
It was always a part of the plan that Jesus Christ would remedy this state (only the fully-divine Christ being incarnated, dying and Himself being resurrected could accomplish this) - and that Jesus would by his mystical work enable resurrection, and thereby restoration of our wholeness.
Why then did the advent of Christ also bring Hell - a place of tormented rather than 'demented' souls as in Sheol/ Hades?
Because Christ brought post-mortal awareness - we must choose to accept Christ's gift of resurrection and a post-mortal eternal life as Sons and Daughters of God, siblings of Christ - of working towards higher levels of our divinization. And to chose this entails to remain aware in the post-mortal state.
Therefore, Hell is Sheol/ Hades PLUS post-mortal awareness: Hell is Hades but ALSO to know Hades - to know that we chose Hades above Heaven and to experience meaninglessness, purposelessness and existential alone-ness.
(Approximately) Hell = Hades + Insight