Perspective of free-will vs God's Sovereignty.
This is an age old question, that if God is all good, then why does evil exist in this world? As an ancient Greek philosopher said,
“Is God willing to prevent evil, but not able? Then he is not omnipotent.
Is he able, but not willing? Then he is malevolent.
Is he both able and willing? Then whence cometh evil?
Is he neither able nor willing? Then why call him God?”
― Epicurus
In a recent movie "Batman vs Superman", Lex Luther said, "If God is all powerful, then he cannot be all good. And if he is all good, he cannot be all powerful.". This is a question that many asks and find it hard to answer.
I do not perceive to know the answer to this deeply profound question But at least I know this is what the bible says, and it is hardly an answer that would satisfy those who seeks justice in a fallen world.
Based on Romans 9 :
v15: For he says to Moses, “I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion.”
[For will have Mercy and Compassion on whoever he choses]
v16: So then it depends not on human will or exertion,[fn] but on God, who has mercy.
[This has nothing to do with human achievements which is already established as nothing from Chap 1 to 3 ]
v17:
For the Scripture says to Pharaoh, “For this very purpose I have raised you up, that I might show my power in you, and that my name might be proclaimed in all the earth.”
[He hardened Pharoah's heart so that his power might be shown, and his name may be proclaimed]
v18: So then he has mercy on whomever he wills, and he hardens whomever he wills.
[God needs no reason to show mercy or wrath to whomever he wills]
v19: You will say to me then, “Why does he still find fault? For who can resist his will?”
[The common human response will be, "That is not Fair"]
v20-21: But who are you, O man, to answer back to God? Will what is molded say to its molder, “Why have you made me like this?”Has the potter no right over the clay, to make out of the same lump one vessel for honorable use and another for dishonorable use?
[Unfortunately, the human race does not have the right to question his creator.]
v22-23: What if God, desiring to show his wrath and to make known his power, has endured with much patience vessels of wrath prepared for destruction,in order to make known the riches of his glory for vessels of mercy, which he has prepared beforehand for glory.
[The Sovereignty of God is such that, all things good or evil, weak or strong, happens and exists in this Universe, for his glory. All things are created through Him (Col 1:16) and nothing exists without Him (John 1:3). We just have to trust that God is Almighty, and he is All Good (Rom 1:19-20)]