Like the title says. Was there any sort of law or convention keeping wealthy Republican Romans -aristocratic or Equestrian- from letting their inner Ramsay Bolton loose on some slaves? If such a individual wanted to torture and rape some people, or play a little 'Most Dangerous Game' on his land with some slaves, would there be anything keeping him from doing it asides social conventions?
What you're talking about is pretty much the story of Vedius Pollio, a rich equestrian master who was famous for his "excessive cruelty" to his slaves. Many Roman authors like Dio Cassius, Seneca, Tertullian or Pliny report that he had a fishpond full of lampreys (moray eels) in which he threw some of his slaves to watch them getting eaten alive. One day while Augustus was visiting, a slave broke a crystal cup, so Vedius Pollio immediately ordered him to get thrown to the lampreys (moray eels). Augustus found that so revolting that he ordered his guards to break every single cup in the house. Vedius then had to forgive the slave (he couldn't at the same time punish him and let Augustus do far worse). But otherwise he was not punished in any way for abusing his slaves. Except that at his death he was so infamous that his Roman villa was destroyed. So not being cruel to your slaves was more of a reputation and honour thing than a legal concern.
EDIT: Note that the source also explains his villa was destroyed first of all because it was seen as too luxuous, rather than because of Vedius' cruelty.
EDIT 2: Thanks to /u/XenophonTheAthenian for correcting the translation - the animals were more likely moray eels.
This isn't exactly what you are asking about, but might be of interest. Martial (Marcus Valerius Martialis) published epigrams between 86 and 103AD (i.e. during the Principate), often containing social commentary. One I remember is:
Why did you cut out your slave's tongue, / Ponticus, and then have him hung / Crucified? Don't you realise, man, / Though he can't speak, the rest of us can? - Martialis, Book 2, Epigram 82
I'm afraid I can't find my Latin copy. If I interpret this correctly, it would mean that "Ponticus" could get away with murder of a slave at this period, but had to dress it up as punishment rather than sadism.
EDIT - /u/HotterRod has given the original below - it seems that there is no mention of the slave being killed.
I would to piggy back on this question and ask the same scenario in the united states. Was there anything to prevent an American master from doing the same?
81 BCE unjustified killing of another's slave was a crime.
Also brutal treatment of slaves could result in disapproval from censors, which could result in a legal disgrace.
Eventually Hadrian creates several laws to protect slaves, but that's not until the empire.
Source:Textbook on Roman Law 3rd Edition
Slaves often had to be tortured before their testimony were regarded as true. This does not answer your question directly, but it gives some insight into the Roman view on torturing slaves.
Read more about it in this thread: https://www.reddit.com/r/AskHistorians/comments/28fwwv/did_roman_slaves_have_to_be_tortured_before_their/
great question, i wish we had more resources on this subject. anything in regards to the emperors?