There are many people today who believe in God, but say there is no devil and no evil.  They say that everything that happens in this world happens because God wants it to and what we think is evil is just part of God's plan that we simply don't understand.  Some have even wrote books on the concept. 

I don't agree.  First, it should be obvious that we live in a world of duality.  Hot and cold, male and female, yin and yang.   Because of this dual nature of the material universe, you cannot have one without the other.  Second, if everything that happens, no matter how horrible it appears to us, is happening according to God's plan, than we have no free will and everything is predetermined.  There would be no reason to strive for better in the world and in ourselves. 

I believe it is worthwhile to improve ourselves and the world because we do have some free will and can choose to serve God by making the world better or to serve the devil by pretending that everything is fine with the world just as it is.