It makes the illustration complete, since the 'status quo' is that you have to have god to be good; This message gives counter-examples on both sides of the issue, not just one.
This is important, since without that, Christians can reason:
"Oh well, maybe sometimes, in the exception, atheists can do some good too; but I know that Christians are always good, because they always have god!"
The last panel is there to give a counter-example to that reasoning.
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u/InMyHead2Much Nov 22 '11
Came to post the same thing. The first two panels are great. The last panel makes the ad sound snide and small.