It’s kinda odd in retrospect. There are many words to describe Romulans… but violent isn’t really amongst the top ten.
Gene Roddenberry envisaged the Romulans as Star Trek’s version of Communist China and the Klingons as its Soviet Union. In Making of Star Trek, he and Stephen Whitfield describe the Romulans as “highly militaristic, aggressive by nature, ruthless in warfare.”
That’s from the man himself. Not sure where you got the impression that romulans aren’t violent.
I’m familiar with that part of the lore.
It’s more like how the Borg are described as an unstoppable unrelenting all powerful force… and are stopped, relent, and are devoid of power. On paper they are one thing, on screen they are another.
With the Romulans, they tend to outsource the violence. Pit party A against B, then clean up after. Practically scavengers. Klingons, Jem’Hadar, and Hirogen I’d more readily describe as violent.
