If Romulans so deceitful and vicious, how can they made any allies?

In the middle of "The Next Phase" a Romulan scientific officer decides to destroy the Enterprise, even though Enterprise was the only ship to respond to their distress call and actually have saved Romulan vessel (and around 100 Romulans) from an inevitable destruction.

I imagine, that attempting to destroy a ship (or kill a person), who has just saved you from a certain death would be the most visious and scurvy behavior among entire Star Trek universe. That could only be comparable to Borg (correct me, if I'm wrong). Are Romulans really "that bad"?

How could they make any allies with any other species, with such attitude? I'm not only saying about Klingons, for which (again, if I'm not wrong) such behavior is most disrespectful and we know many examples of Romulan-Klingon ally. I'm also saying, that they wouldn't be able to may any kind of ally. If you're ready to destroy the one, that is saving your live, then you're als able to destroy your ally in any kind of situation -- i.e. you're completely unstable and unreasonable as any kind of ally.