Henry Clay Work was from a family of abolitionists. The family home was part of the underground railroad. Kingdom Coming was written in sympathy for the enslaved and gives them the upper hand in the song.
Yes, it seems that the author had nothing but good intentions in mind. This used to be a regular part of my presentations on the songs of the Civil War. One can clean up the dialect but "darkey" can't be ignored. The last time I sang it was in a lecture of 19th century songs for a graduate class of students of history. The song was presented to show that while the writer had good intentions, he still employed offensive language common in his day. The professor agreed with the song's inclusion and yet even in this context, some were offended. The melody is quite appealing and I play it on the fiddle. But I will sing it no longer.