Atheists: Your all about logic and reasoning and evidence and stuff right?

How can you completely denounce any god/goddess/deity/etc. as completely bs? I know there is a lot of logic to disprove their 'realness' but there is a lot of logic on the other side of the argument as well. I'm not ranting and stuff I honestly want to know.... and other random question, what is the difference between atheists and agnostics?