From what I've seen on screen, I definitely have to echo C.E. Evans
point about there being biological aspects to their violent streak, in spite of the Romulan's presentation.
To reconcile the two, I have always been of the belief that imperialism is a distinct idiosyncrasy of the Romulans as a people, relating primarily to their chosen way of life and of organizing their society. But there is nothing on-screen stating that this confers any behavioral condition upon them individually. They could very well have embraced some form of emotional control (albeit obviously less strident), and it would be the rational course of action if indeed their race (Vulcans writ large) is beset with abnormally erratic emotions.
Rather than reject emotional control, we've only ever seen that the Romulans reject logic
specifically, and especially where it regards how they govern their society and their lives. However, the two states of existence (emotional suppression and violent imperialism) are not mutually exclusive. The coexistence of these approaches can, IMO, entirely explain the way the Romulans we've seen have been depicted.
The Vulcans seem to have chosen to integrate the pursuit of logic and rejection of unreasonable use of violence, with its attendant emotional suppression, into the way they run things on their planet. Their way of life is highly dependent on its strict adherence. Career, family, government and moments of crisis are all engaged through the purity of reason. It's systemic and a rather drastic leap from merely suppressing emotions.
They are the central tenets of Surak's teachings and it was probably this
that drove the Romulans' ancestors to seek another path.