I truly appreciate the rational nature of this discussion. From a Christian perspective one might agree that God does not create evil, but even though evil (or wrong doing) exists because of man's freedom of choice, God will still accomplish his will. Evil or the absence of light or goodness is a different philosophy, but it does not conflict with the Christian perspective. Evil exists, but different cultures-different societies view evil differently. For example: in American culture marriage to and intercourse with a 12 year old girl is socially unacceptable. However, in other countries being a child bride is the norm and even respected.
Finally, evil does exist, and while some of its existence might be cured by love, medicine, or counseling, some broken things cannot be fixed.
What is the origin of evil? If the origin is the devil, then the existence of the devil does not disprove the existence of God. If the origin of evil is humanity, then the existence of humanity does not disprove the existence of God. I think the fact that evil exists is proof that God gave humans free will, not a way to try to argue against God's existence.
The term "evil" is an adjective created by humans to describe abhorrent and despicable acts. Abhorrent and despicable acts are a product of bad upbringing, mental illness, and sometimes even dysfunctional parenting. Hence, there is a lot of HUMAN and not SPIRUTUAL background to what some call "evil". "Evilness" then is a problem that has a very human solution. Therefore, no. The existence of "evil" is not a correlation to the possible existence of an entity called "God".
If such an entity existed you would have to futher expand your question to "why is it letting evil take place?" For that, you will get a myriad of different (and often nonsensical) answers.
What I do believe is that the existence of evil has more physical and psychological causes than anyone would care to admit, and that with proper psychiatric care, good nurturing, and less nonsense we can raise individuals that will grow up to be kind, generous, non-violent, and perhaps even "normal".
This is, of course, a matter of opinion. However, in my opinion, the existence of evil should not call into question a belief in God.
Let us assume that God wants us to be good and wants good things to happen to us (I know that this is a big assumption, but it is already assumed in your question). What does it mean to be good? To me, it means that we have a choice between good and evil and we choose good. If there were no evil in the world, the choice of good would be meaningless just as winning means nothing if there is no way to lose.
So, if God wants people to be good, there must be the possibility of something other than good so that the choice that people make is a real choice.
satan the devil is the origin of evil. he has questioned Gods authority and that we can govern ourselves. yet we see the conditions of the world going from bad to worse so this should be proof to us that he is the father of the lie.
Evil is nothing in itself it is only the absence of good. It has no identity on its own. As darkness is nothing but only the absence of light.
Evil is not created at all by God but it is only the absence of good.
Islam and concept of Evil
Let us remind the reader that Islam defines evil only as a shadow created by the lack of light. It is not a positive existence in itself. We can imagine a source of light (a lamp or the sun), but we cannot imagine any object as a source of darkness. The only way in which an object becomes a source of darkness is through its ability to obstruct light. Likewise, it is only the absence of goodness that constitutes evil. The grades of evil are only determined by the opacity of the obstructing medium.