Before you jump down my throat, understand that this is the sort of ridiculous statement told to me quite often by my relatives: "Jesus Christ stopped the evil pagans from burning their slaves after dark for light." They tell me this with this look in their eyes that says they're sure this should be a convincing argument for why I shouldn't be pagan.
Funny, I may not be Christian, if I ever really was, and I feel like I should spontaneously combust when I go near churches, but I don't recall that scene in the Bible. Apparently, this is coming from "Quo Vadis," which seems to be a bit of long winded romantic fiction from a little over a hundred years ago, set in Nero's reign. It's not what I would consider a very unbiased source of information, but apparently this has become fixated in their minds as characterizing the treatment of everybody everywhere towards like...I guess everyone before Christianity took over and supposedly we weren't assholes to each other anymore (lolwut) until the evil Muslims were a thing? (/s, in case that wasn't obvious...)
Personally, to start with, and setting aside any moral argument, this just sounds stupid as hell from a purely economic standpoint; burning slaves after dark just sounds like a terrible waste of economic resources, and I couldn't imagine that the shrieks of a burning man would be a very relaxing atmosphere for reading or filing taxes. Not, at least, a thing the average citizen would do when a candle would be far cheaper. Not to mention, from what (little) I understand about slaves in the Roman empire, they weren't in the strictest sense under the same rules as, say, slaves in the American South. There was some opportunity to leave slavery, and there was some legal recourse.
I know the situation was far more complex than simply writing it off as "the pagan world was more or less dark and vicious before Christianity," but I don't know very specific particulars, and I end up just rolling my eyes and having another drink. I try to talk them down from the pulpit whenever I can field a counterargument, but this part of history isn't one I'm extremely familiar with, and being able to give them something to track down and read helps.
Knowing that this is a very, very broad question: Can anyone help me better characterize pre-Christian treatment of slaves, especially as regards the Roman Empire? This is likely to vary widely according to place and time, but since this is story is set during Nero's reign, what were the circumstances of Christians then, in particular?
Tacitus says that, for a time, Nero scapegoated Christians for the fire of Rome and that he killed them in a number of nasty ways, including burning them alive as torches at night:
Consequently, to get rid of the report, Nero fastened the guilt and inflicted the most exquisite tortures on a class hated for their abominations, called Christians by the populace. Christus, from whom the name had its origin, suffered the extreme penalty during the reign of Tiberius at the hands of one of our procurators, Pontius Pilatus, and a most mischievous superstition, thus checked for the moment, again broke out not only in Judaea, the first source of the evil, but even in Rome, where all things hideous and shameful from every part of the world find their centre and become popular. Accordingly, an arrest was first made of all who pleaded guilty; then, upon their information, an immense multitude was convicted, not so much of the crime of firing the city, as of hatred against mankind. Mockery of every sort was added to their deaths. Covered with the skins of beasts, they were torn by dogs and perished, or were nailed to crosses, or were doomed to the flames and burnt, to serve as a nightly illumination, when daylight had expired. Nero offered his gardens for the spectacle, and was exhibiting a show in the circus, while he mingled with the people in the dress of a charioteer or stood aloft on a car. Hence, even for criminals who deserved extreme and exemplary punishment, there arose a feeling of compassion; for it was not, as it seemed, for the public good, but to glut one man's cruelty, that they were being destroyed. (Annals 15:44).
This is Christians, not slaves, and Tacitus portrays it as something that was seen as excessive by Roman standards, not normal or typical, but the the claims the OP is talking about may be a garbled reference to Tacitus.
Obviously the claim that Romans burned slaves for light is absurd for multiple reasons, not the least of which being expense. That's like people burning their cars for light.