the Romulans are scarier because they're more cold-blooded.
Plus, there was this spooky relationship with the Vulcans and the split. I think Romulans and Vulcans were basically cousins. And I guess this showed that logic could be destructive, unethical or at least disregarding of others, accepting war as foregone conclusion, accepting death as foregone conclusion, saying what difference does it make -- well, it sure as hell matters in the meantime! -- or things which sometimes repel us, sometimes attract us, and thus get an emotional reaction and make us feel uncomfortable.
I remember Kirk almost being seduced by a female Romulan leader even though he was playing a role in a complex plan. And I don't mean simply sexually.