This question is prompted by an earlier question on Roman law, which seemed to suggest that only treason was punishable by death for citizens. I did ask this as comment, but have got intrigued. I am ashamed to name my "source", as it was a murder mystery set in Rome, where it states specifically that the penalty for patricide - being considered a particularly impious crime - was to be flogged, tied in a sack with various unpleasant animals, including a snake, and rolled into the Tiber! Is this arrant nonsense? I'm quite prepared for the answer "Yes!" grin
earlier question on Roman law, which seemed to suggest that only treason was punishable by death for citizens
Not really. The point is that Romans belived that public execution is also a too deep shame for any citizen. That means that they practised:
- secret executions (e.g. laqueus, i.e. execution by garrote)
- murdering not being "execution" de jure, e.g. burying vestals alive, starving prisoners to death etc.
- military punishments outside of the pomoerium (totally different jurisdiction!) - think of decimation etc.
- pater familia might kill the criminal without any need for public trial
- actually public executions for the persons found guilty in too shameful deeds from Romans' point of view. Note that they usually hid doomed person's face by some veil, which is to remind of sacrifice.
As the reasons of the latter they usually say of treason, incest and patricide. Yet note that the term "treason" (or, to say it right, "perduellio") was quite a special thing - it was a crime against the state and the order, not just "a treachery" in a modern sense.
Livius I, 26
The dreadful language of the law was: “The duumvirs shall judge cases of treason; if the accused appeal from the duumvirs, the appeal shall be heard; if their sentence be confirmed, the lictor shall hang him by a rope on the fatal tree, and shall scourge him either within or without the pomoerium.”
Here Livius says about early Republic hero Publius Horatius who killed his sister. As she was a free citizen, then public murdering without a lawful reason was considered as a crime against the Roman state itself!
On the matter of "patricide" and the punishment for it called Poena cullei, it's better to read the corresponding article in Wiki, which is quite comprehensive: https://en.wikipedia.org/wiki/Poena_cullei