Supposedly, Christianity has a problem explaining the existence of evil in the world given the existence of God, who is good. I don't see the problem at all, so let me do my best to explain why. Primarily, I think the bulk of the problem arises from not understanding what good and evil are. So, let me start there.
God is good, Jesus said uniquely so. God is good, and nothing but good, the very essence of what good is. Evil, on the other hand, is not a thing so much as it is a description. At best, it is understood negatively, by what it isn't. Simply put, evil is that which is not good, much in the same way that darkness can be defined as the absence of light. Light is the something, darkness is the privation of it. Evil is a privation of God, an incidence of something God is not in, so to speak.
In being good, then, God can have no part of himself that is not good, or not God, or else he would be a contradiction. In the creation account in Genesis, the word translated "good" has the general sense of beneficial, so I think we can say by extension that good is that which comports usefully with the beneficial purpose of God. Since God can have nothing in him or about him which is set at cross purposes against himself without being a contradiction, good is that which is in accord with the purpose and will of God and evil is that which which is opposed to God.
As a consequence of this nature, it is possible for a thing to be both good and evil at the same time. A thing meant to thwart God most definitely comes from an evil intention on the part of the would-be thwarter, but in God's providence it may actually further his agenda of good. With that in mind, one might surmise that things which seem to us to be detrimental to life, to which our guts recoil, and which we see as nothing but evil are actually not evil from God's point of view. God intends such things for good or such are not from God who is only good.
And yet everything which exists, exists, ultimately, as a result of God's doing. Why would one who is only good, allow the existence of that which appears so obviously non-beneficial, so outrightly evil? Even if that could be written off as merely a momentary or passing appearance (to us) which served a larger good (from God's perspective), that wouldn't make those moments of evil less evil. So to me, such an explanation seems disingenuous.
What I mean is that evil is evil, even if just for an instant. If God is, then all instants are his, so why any evil whatsoever? God would not be the god who is good if there are flashes of evil in him, bits of momentary lapse where his "other side" is displayed, even if that is not his general character. No, it seems to me the existence of any evil whatsoever does not comport with the existence of an almighty god that is good; and yet, it is my unshakeable belief that the Sovereign Lord does exist and is nothing but good, and that evil exists here and now.
How can that be so? It must be that a good that God did, that was only good, must have given rise to evil. Having given rise to evil that which was made good by God, but which had become evil, has been allowed to remain so for the good that God continues to do in regard to it. So, present reality consists of the good that God, who is only good, continues to do while that which was created good, but is no longer so, continues to express it's evil.
Since evil, by definition, is that which is against God, that good that God did which became evil must have been able to act in a way that opposed God. The only way that would be possible would be for that something which God made good, but which became evil, to have had the ability to act freely, uncompelled, without necessity, as does God. Since God is good, there could be nothing evil about making something commensurate with God, but in doing so, the possibility of opposition, and therefore evil, would be intrinsic to that act.
What should begin to be clear from this discussion so far is that there could be no evil in a world in which God's will was the only one that determined things. If God determines everything that is and occurs, then evil could not possibly exist. For evil to exist within a deterministic framework, God himself would have to be evil. Since God is clearly not evil, and yet there is evil in this world, this world cannot be a deterministic one.
And there is more to delve into...