If you have been an atheist for a certain amount of time you will find that some theists will say something like "You just want to sin." There are so many things wrong with this sentence like, who says its a sin (morality questions). In this post, however I want to focus on people behaving badly. When I say behaving badly, I will be referring to what us normal minded (relative term) believe is bad. For example a terrorist, a murderer, a thief, a conman etc. are all people I would consider bad. This concept of bad for me comes from what is best for the community and society I live in, and I think most people would agree the examples I have listed above would not be beneficial for most if not all societies.
The fact is some people are just bad, there are reasons in their psychology why they are this way but no god will make them good. However, theistic beliefs give some bad people a reason to do their bad
things as they know they can claim some type of good or forgiveness which justifies
their actions either in their eyes or the eyes of others. In a similar way by offering up religion as a cure for bad people, we allow bad people to get away doing bad things under the guise of forgiveness or religion. Just think about all the catholic priest paedophiles who have been getting away with their behaviour for years due to corruption in the church and denial by theists.
Another bigger screw up is that by allowing bad people to live with the so called religion cure is that it can perpetuate into far worse atrocities. A horrible example of a religion/cult getting out of control is Jim Jones and the Jonestown massacre. Another example would be the 9/11 attacks on the USA, which was carried out by delusional Islamic terrorists.
Bad people exist, and they will exist with or without religion, but we do not need to give them more excuses. Remember, religion does not stop bad people doing bad things.