I can’t top the excellent blog post about this painting, so I’ll just add a link.

http://blog.interiorpaintsecrets.info/st-mercurios-killing-king-oleonus-st-mercurios-killing-emperor-julian-the-apostate/

If you want to follow my extended review of Decline and Fall of the Roman Empire from the beginning (and who wouldn’t?) it starts with Augustus founding the empire.