Politicians are expected to release their tax returns as a way to ensure that they are not engaged in a clandestine activities, which usually means bribery or other kinds of corruption to the loss of taxpayers. So this is only to be expected that a lifetime politician like Mrs. Clinton releases hers.
Mr. Trump is not a lifetime politician, his income could not have anything to do with bribery, or at least accepting bribes. He has earned his possessions in his business life, and he never had any power that would make him a corruption target. So he's in his right not to release his returns, even if we'd like to see them (and who wouldn't).
So get over it already, he may or may not release these returns, but he's ultimately right: it's none of our business.
Trump gave $100K to the completely corrupt Clinton Foundation. Although you have a point about him perhaps being less of a career politician than Clinton, and a point that his tax returns will therefore be cleaner than hers, he's far from uncorrupted.
The sole time he openly attempted to access the power of government, it was in an attempt to horribly abuse the worst SCOTUS precedent in recent history, Kelo v. New London(2005). This decision allows for a phenomenon called "eminent domain abuse."
Now, true libertarians all decry ANY use of eminent domain, believing that all goods and services can be traded voluntarily, on the freest-possible market.
That said, even statist-collectivist mainstream Republicans hated the Kelo decision, which allows the government to confiscate privately-held real-estate(including lived-in residencies! ...our homes! ...our very shelter!) , and deed it over to A PRIVATE ENTITY solely because that entity offers to pay more in taxes than what the current owner pays!
That's essentially communism, and Trump tried his best to use that communistic power to steal Vera Coking's home.
This means that Trump is unprincipled, and anti-property rights. Property rights form the legitimate basis of the Republican Party. Without a robust defense of property rights, Trump cannot be considered any more "republican" than Mao Tse Tung or the Russian communists who confiscated houses and real-estate from middle-class capitalists and entrepreneurs (like Ayn Rand's father).
So, Trump is against other people's rights to their own real-estate (when it suits him). What about other property rights? Well, he's against drug rights. (He completely doesn't believe we have any right to our own bodies, nor to decide which, if any, recreational drugs we use. ...Unless it's a stupid and destructive one like alcohol or cigarettes; then, and only then, he's in favor of treating that right as a privilege to be granted by the all-powerful state.) Of course, he claims to be in favor of gun rights ( but of course, all the abusive, extra-legal tools and practices that the DEA uses, the ATF eventually adopts, rendering Trumps soft support for gun rights meaningless).
So, in short, Trump is an unprincipled fucking idiot and a hypocrite, whether he reveals that with his tax returns or not.
If you want someone who's remotely "good" you have to vote for Darrell Castle or Gary Johnson.
The point is, he's certainly far from the receiving side of bribery, so what would his tax returns reveal? That he's rich?