Thanks for the question.
You are talking about the problem of evil that has been difficult for us to comprehend for as long as humans have been thinking.
The fundamental answer to the question:
Why would a good God allow evil to exist and thrive?
lies in the reality of free will. Think about a reality where God did not permit free will.
Every sinner would have to be destroyed on the spot, because sin is evil.
- If you ask, why couldn't God just make it so that they couldn't sin?
then you would be asking God to make automatons that are only capable of doing what He wanted them to do, which is contrary to the human nature, which He created in His Own Image . . . having free will as a dominant attribute. That is the great power of God, to create a new free subject, capable of even rejecting Him so the questions really become:
- Why does He permit this really evil stuff to keep going on?, and
- Why doesn't He just pull the plug on it?
That is where Jesus comes in. He gave us lots of answers. He told us that God lets the weeds and the wheat grow together, and He doesn't pluck out all the weeds because he wants to save some of the wheat that could get lost in the plucking (cf. Matthew 13:24-30). There are many stories and parables that Jesus taught us to give us an insight into why God does the things He does but above all things, He sent Jesus to become one of us; to pull us out of this evil mess and transform our brokenness, so that like God, we might become fully good, choosing love over evil in all things.
That is what our freedom is meant to do: allow us to love as God does, freely and wholeheartedly.
Lastly, God does plan to pull the plug eventually, and Jesus taught us that too. That is too big a subject to go into here, but suffice it to say that justice belongs to God and He will exercise it.