It seems to me that a myopic view of the equality of souls is too often adopted and may be as bad a doctrine as that which prescribes a deprecating prejudice, condemning people for their current perceived faults.
True, it is, that each soul is precious and equally so.
True, it is, that it is difficult to judge another's potential by his current faults. We may fail to see clearly, or the one judged may repent.
Further, it must also be kept in mind that judging how great we are is important. (That is, it is important to know how we stand before God.) However, it is much less often that we need to know how great others are, and I am unaware of a compelling need to be able to compare one's self with others in this respect. Nonetheless, to deny that one soul may be better than another to some extent denies that there is a better state which a soul might strive for.
Christ is better than me and I am better than Satan or Cain. I expect it not heretical to state that I, and each of us, are better than one third of the host that was in heaven premortally. This ought not to be a cause for pride, but rather should be a cause for us to look forward and strive to be better than we are now.
Cain's soul was just as precious as yours or mine, but it was wicked and he corrupted himself rather than bettering himself.
This is perhaps similar to the idea of being equal in opportunity, but not in circumstance.