Lt. Cheka Wey wrote:
A democracy that has colonies where they treat people as subhuman...
A democracy that is waging proxy wars against peacefull nations...
A democracy that gives financial aid to bad dictatorships...
This sort of thing. I cannot recal any from the top of my head. It would be nice to see some more complexity in Star Trek's politics.
I don't think it makes sense to throw these two notions together. If you lead wars and use slaves you are not a democracy. Democracy without the rule of law is mob rule. Not that the mob ever has much interest in waging wars or owning slaves. Such societies are always oligarchic, ordinary folks are not rich enough to own slaves and gotta die in the wars.
Let's take Romulus. The little we saw in Unification indicates that it is an authoritarian society, ordinary people fear those who wield power. In addition to that it is probably very dogmatic like Vulcan society, strong emotions gotta be channeled and the Romulans do it via conquest. If you want your society to be stable the method of channeling of emotions, be it Romulan imperialism or Vulcan orthodoxy, must not be questioned.
Whether the Romulans have a Senate or not matters little. Having formal democratic institutions doesn't make you a democracy yet.
Except you can have an society with an elected government at home and rights for citizens at home and have a imperial policy that doesn't extent that policy to its colonies. The British Empire was a democracy for a lot of its history, the monarchy slowly lost power and the elected Parliament gained more, but they still had an imperial policy. I don't think anyone would say the UK was a dictatorship, but it certainly did not grant the same rights to the people in their colonies as it did people at home.