I have no evidence of God, therefore no belief
I was a faithful Christian for many years, but it just didn't work. I prayed and did everything I was supposed to do, and believed with all my heart. Nothing happened. In fact, I got more depressed than ever. I do not believe in God because I have no evidence of a God. I'm not even sure if Jesus existed or if he was a fabrication out of the cult of Mithra. I cannot have a personal relationship with God or Jesus if he does not communicate with me. So far all my prayers and efforts have been met with silence. As far as I'm concerned it's just wishful thinking. I was told that if I prayed hard enough, God would talk to me and everything would be wonderful. That was baloney. You cannot make yourself have a subjective experience just by wishing for it. Besides, if you did, it could be explained in terms of brain chemistry.
I do not believe that Jesus died for our sins, because I think we have to deal with our sins ourselves, forgive ourselves and try to do better. There is no God standing over us watching us like a cosmic cop.
I don't believe in an afterlife because I have no evidence of it. It makes no difference in my actions. There is an article at beliefnet by Alan Dershowitz where he postulates that atheists are more moral than Christians. Why? Because Christians do good because they hope to be rewarded in the afterlife. Atheists do not believe in an afterlife or a reward. Atheists do good because it's the right and moral thing to do, without hope of reward.
We should do good for others without hoping for a reward in heaven. We should do good merely because it's the right thing to do, whether there is a god or not.
So chew on that for a while.
The Jews do not dwell on heaven, because they believe in doing good while you are here on earth, and helping other people. I think that is a very good philosophy, because I do not know for sure that there is a heaven or an afterlife. I know I am here now and as far as I know I only have one lifetime here, so I better do good while I am here.
You asked why Christians would be so cruel to others. I believe they suffer from low self esteem from internalizing the doctrines of original sin, you'll never be good enough, only Jesus was perfect, blah blah blah..... So to reduce their pain, they have to turn on other people and attack them, blame them, judge them, condemn them to hell and so forth. It takes their mind off their pain. They also think that they are going to heaven and every body else is going to hell, and they'll be glad to tell you why you're going to hell. They have simple minds and simple answers that just don't work in today's society.
They should not have bought into those horrible abusive doctrines anyway, but that's not my business to tell them.
Obsessive-compulsive behavior of any kind can be life destroying. Usually compulsive people indulge in repetitive behavior to take their minds off their pain, and people who are obsessively praying and worshiping God are no exception.
Read "Healing the Shame that Binds You" By John Bradshaw, Ph.D. He is a psychologist who was a priest for several years. He has studied how religious belief is maladaptive for people and causes them unnecessary torment.
I hope this explanation helps.
Peace to you.