Suppose we could, at no cost to ourselves, prevent the painful death of a deer in a forest fire. All things being equal, this is what we would be obligated to do. Now we might decide that we have to consider our own safety, or our own limited abilities, in determining whether to rush into a burning forest to save a deer. But, if we could save a deer by pressing a button on our computer, woudn't we do so? When we see some unfortunate thing happen, like the disaster in Haiti, we certainly are inclined to hold anyone accountable who could have prevented the tragedy at no cost and failed to act.
But what about God? God had the ability and the knowledge to prevent any catastrophe that happens. So if He knows about them in advance, he has the ability to prevent them (this is entailed by omnipotence and omniscience), the only conclusion, apparenlty, is that he doesn't want to prevent them.
Suppose we were to find out that a multi-billionaire, who has recently died, had a room where he could go with all the latest computer equipment, and he had the power to stop any of the world's great tragedies of the last 10 years. He could have pushed a button and spared us from 9/11, maybe by causing the hijackers to throw up in the bathroom instead of charging the cockpit with box cutters. He had the ability to send Hurricane Katrina back out into the Gulf of Mexico and away from the city of New Orleans. He had the power to spare Haiti from the earthquake, and the Asians from the 2004 tsunami. But he did nothing. Would we think well of this person? Yet, God is supposed to be perfectly good.
Now I don't buy this argument, surely, but it is important to see the force of it. It's the number one reason atheists will give you for why they are atheists.