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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:
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: βThe 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.β
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