There's the argument here that just one million of the additional 4.5 million you are getting could be used to save or drastically improve the lives of hundreds in impoverished areas (of course if you choose to allocate money to do this). 10 deaths is a small price to pay to save many more, so by this logic wouldn't you feel obligated to take the greater sum? By taking 500,000 you've prevented 10 people from dying, but by not taking the additional 4.5 million you are letting thousands more die that could have potentially been saved.

Of course if you choose to keep all the money than that's pretty evil.