Christianity gives two accounts of how evil is possible: a.) Whatever is other than God is not identical to its goodness and so is (logically) capable of existing without it and b.) God allows them.* It also gives one account of the order of evils: angelic, original, actual (it also gives some account of the order of motivations and actions). These explain how evils are possible and what order they came in, but this is not at all what most bothers us about them. That there is no logical answer to this, whether in theism or outside of it, is just the way things are.
*Just what this allowance involves is unclear. We need more thought done here to figure out whether actual evils might be allowed for the sake of possible goods (that may or may not happen) or whether evils in general have an order to some particular greater good (no sin, no Incarnation, etc.) but need not have a straight-line causality between any one evil and some greater good, or to what extent evil arises out of respect for the order of creation, which is inseparable from things that have no rational point (chance, action in vain, bad luck, the higher depending on the lower, etc.)
If “allowance” is a way of talking about how no evil needs to be meaningless it has some value, but if understood as making evil a sort of currency that God uses to purchase goods it is ridiculous and explains nothing. The Patristic dismissal of redemption as a “debt paid to Satan” is in line with this.