1. I define evil as opposing God, creation and The Good - 'the good' existing only within God's creation; and God's creation being a consequence of Love (initially, the love between our Heavenly Parents, which was the motivation for creation). God created The Good - so, to oppose God and creation is to be evil. And evil is the rejection of Love.
2. All Beings, including all Men, have existed from eternity - initially as primordial Beings, later these primordial Beings were procreated as sons and daughters of God, in the form of spirits (without bodies).
3. Evil in the universe comes from Men, and was always present from eternity. It could be thought of as a disposition, a character trait: the trait of Pride, which is broadly in favour of the self and its satisfactions at the expense of others; and specifically therefore against being a part of the familial ideal of God's creation harmonised by love.
4. When we became children of God we lived initially as spirits in Heaven, with very little agency - we were immersed in God's Goodness and the life of Heaven - therefore (somewhat like young children) we had very little capacity for expressing the evil that was within us.
Probably all pre-mortal spirits (excepting Jesus) will 'contain' evil, but they will not always express evil - because all are immersed-in Goodness and without a boundary between God and himself or herself.
(Analogous to a young child immersed in the loving kindness of a perfect family - in such conditions, few young children will express evil.)
However, some pre-mortal spirits had so much evil in them - from eternity - that (even with the limited agency possible in pre-mortal spiritual life) they opposed God, creation and The Good. They chose to express this opposition in Heaven; and so were cast-out of Heaven. These were Satan and the many other demons, now eternally active (because eternal Beings) outside of Heaven; and including this world.
5. In order to develop towards being fully divine Children of God, we had to become more and more fully agents; and part of this development is incarnation into this world.
By incarnation (getting 'solid' bodies) we became separated from God (incarnation can be seen as an increased boundary between our-self and God); and for the first time then able to express our distinctive selves, including the propensity for evil.
Thus Men (each of us) brought primordial evil into this world, and by being-incarnated enhanced our agency; and this increased agency 'unmasked' this always-present evil, and enabled it to be expressed.
6. We (here and now, in this world) each find ourselves living adult lives in a situation of evil from demons, from other men, and in our-selves; in which situation we must choose either for or against God/ Love, creation and The Good.
Those who, after this life, choose For God and wish to participate in the work of creation will (thanks to Jesus Christ) be resurrected into Heaven; as wholly-Good agents, like God.
Those who choose God and The Good but do not wish to participate in creation will be allowed non-participatory, non-agent union with deity impersonally known (approximating to the pre-mortal spirit condition); and dwell in a 'timeless', static state of abstract bliss or Nirvana - within the scope of Heaven.
Those who reject and oppose God, who are active against creation, have chosen to reject Love (this called the sin of Pride)... will thereby choose to enter one of the many cut-off personal Hells, which each will 'rule as his own supreme deity.
7. Thus evil is only present in this world because Men bring it here; as well as because of demonic presence and activity. Evil is in all Men from eternity (but to greater or lesser degrees, and of different types and emphases); and this evil is expressed because of our agency.
Also, this agency also allows us to choose to express evil or not. Thus all expressed evil is a choice and a collusion, and the rejection of evil is a refusal of assent to collude. In sum - evil versus good is about inner choice, that is, about collusion versus repentance.
Good and evil are therefore essentially a matter of taking-sides, choosing either for or against God, his creative work and plans; and for or against the basis of creation in Love.
Note; the above is, in essence, my interpretation and extrapolation (and to an extent correction) of Mormon metaphysical theology.