If there were no witnesses, how would they ever find out who killed a person?
And if it was difficult to impossible, what did that mean for the general murder rate of the average Roman?
I asked this question about criminal investigations in the past a few years ago and the top answer by /u/Algernon_Asimov talks about ancient Rome. I'm not positive those answer still live up to the modern standards of /r/AskHistorians, but you may find them valuable nonetheless.