The moral argument for God can be simplified to one sentence: If there is no God, anything goes. This is because the removal of God allows us to take a relativistic approach to morality. Morality, rather than being a set code intended for all people for all time, merely becomes a matter of opinion.
This is why we often hear arguments like "I'm a good person, so if there is a God, He will forgive me." When you don't believe in God, you become the standard. The worst evils in history have been committed by people who believed they were justified. People who believed that even if what they were doing was undesirable, it was still a good thing in the long run. The moment you remove God, you remove the fear of Him, so why would you seek to obey Him? Why would you hate your sin and repent? Why wouldn't you look at God's judgements, which the Bible says are coming to us all, and say that is evil? Why would you believe the only way to escape that judgement is because Christ took it on your behalf? It is sheer pride. Nothing more, nothing less.