Why do 'good' atheists think there good?
Human beings are social animals. Most people feel they are doing "good" when they are adhering to their cooperative nature as social animals in compliance with the roles and expectations society sets for them. Being "good" is often synonymous with conforming to society's expectations and, by extension, the expectations you set for yourself as a "good" member of a society.
"Good" is not arbitrary though, no society/tribe/clan would rule that murder is good since that would be detrimental to cooperation and social order and weaken the society's chances of flourishing or surviving and the individual's along with it.
But there are no guarantee. I'm sure many citizens of the Third Reich thought they were doing "good" by fighting for a maniacal and destructive regime as soldiers in Hitler's armies, or by helping see to it that their Jewish neighbors were deported and killed.
Don't they know that they get there internal thoughts from God?
If a persons sense of "good" and "evil" are coming from God (who should always objectively know what is good or not in a given situation), then how could soldiers from both the USA and the CSA be fighting for what they felt was the "good", and claim God was on their side, when they were fighting for opposite ends.
Even democracy itself comes originally from Christian values....
Simply not true.
Do Atheist know that when they behave thereself and are good where it comes from? Try and think about it!!
Thought about it, and I think what you're implying is wrong.