If people labor under the misconception that there is no hell, no witness to their acts, that no one knows, sees, or takes note - then, if they have no conscience, they will become less than animals.
We see this around us full blast. The fear of hell is not the most spiritual or religious reason for behaving properly, but it is necessary to keep humans human. A higher or better reason is to act in a way to please God - but to get to that point, a person has to at least refrain from serious vice. The false idea that everyone gets the same results, no matter what they do or how they live, or that there is no reaction to anything they do, is an extremely dangerous fallacy.
You reap what you sow - a basic law of the universe that no one can escape from or argue with, no matter what religious persuasion, belief, or lack of belief.
Just because someone may think God doesn't exist, therefore He doesn't exist. Millions of peoples' disbelief in God doesn't affect His existence.
very good point. I've often heard atheists belittle Christians as "immature" because, allegedly, Christians only do good and avoid evil because they are afraid of Hell. And it is true that many of us are primarily motivated by fear of Hell. I know I am, in some areas of my life. Ideally we would all do good and avoid evil out of pure love for Christ. But that's not the case, and we need to start somewhere. Fearing Hell is a good starting point - it spurs us on to repentence and reconcilliation with God. A very wise person once told me, if we waited until we had perfect motivations, we'd never get anything done. It was the best piece of advice I'd ever received.