In Wang Mang's brief but disastrous interruption of the Han Dynasty, he made a number of (in)famous reforms, one of which was the abolition of slavery through the empire. Do was have any records of why he did this? Was it inspired by a deep moral conviction, was it part of his grander socioeconomic strategy, was it some combination of the two, or something else?
Chinese historiography in this period is a significant problem for us. The records that we have for Wang's 14-year reign date practically entirely to the period of the restored Han, and were written to demonstrate that he was a usurper who had never been a legitimate ruler. They obscure his actual policies and motivations, to the point that, while we have some idea of what he did, it is extremely difficult to work out why he did it.
There are two broad schools of thought, historiographically, that seek to explain his revolutionary policies. One, dating to the 1920s, is that Wang was a sort of proto-socialist who genuinely conceived of, and attempted to create, a fairer society with a lower level of inequality. The other, more recent and, in the eyes of most historians of the period, more credible, is that Wang was actually influenced by Confucian ideals and the desire to return China to what he saw as its golden age under the Zhou dynasty.
I wrote an essay about Wang for the Smithsonian a few years ago, and this goes into further detail about his ideas and what we can, and cannot, understand about his policies. If you're interested in delving further, it is still available from my website. But, to respond to your specific query, we simply don't know:
Later the emperor imposed a ruinous tax upon slave owners. It is equally possible to interpret this tax as either an attempt to make slaveholding impossible or as a naked grab for money.