Essentially what you just said is both Biblically incorrect and a manifestation of "mysterious ways."So God doesn't just stop bad things from happening because of a number of reasons.
1) 1st and foremost The world actually belongs to the devil, God is its owner still, but the devil actually rules the world hence why he was able to offer it to Jesus for a act of worship. Its also why people are running around like madmen mostly... The devil will be evicted soon enough but for the time being its his joint.
2) We sinned and took ourselfs out of Gods protection, and since God is well God he didn't just clean slate and start over, plus it wouldn't have answered or covered the challenge/question that the devil suggested That we should be able to decide for ourselves whats bad and good right and wrong.
3) God is allowing man time to see that they cannot rule themselves and under no circumstances will man live peacefully, safely and happily without following Gods instructions. No man can challenge or question that after its all said and done.
4) You have to remember God thinks on a perfect level, we as human are use to being on the top of the food chain with the "best line of thinking" but we aren't perfect nor can we begin to think or reason on Gods level.
The "Devil" is an angel, an "adversary of the faithful" often used by God to test the resolve of "his servants" at least, according to the Old Testament, paying particular attention to the original Greek translations (there's a difference between "ha-Satan" and "Satan").
Not the ruler of an Earth domain.