I'll summarize it below:
First we need to start with "God exists". If He doesn't, then we can't judge good or evil--> everything happen just naturally.
Now, if He exists, why evil exists?
First, keep in mind that :
A. Nothing happen in this world apart from His will.
Note: Don't try to get God off the hook when evil things happen by saying these reasons:
a) He didn't want that to happen
b) He will fix it
c) He couldn't mess with your free will
All these disproportionate God's sovereignty.
B. God is determined to allow human to have what they wanted.
C. Human also have responsibilities.
Let's take example of the greatest evil: the murder of a sinless man, named Jesus. Why God didn't stop this murder happening?
We are sinners. Jesus didn't just die and conquer death, He also
suffered with us: physical assault, accused, hungry, tortured, bear false witness, friends abandonment. Jesus tasted everything even the murder- all are
predetermined for God's glory so that we can be saved.
Conclusion, anything we call 'evil' are still purposeful event.