How is Denethor a tyrant? Yes, he is not the king, but the stewardship is a hereditary title, and as such he is entitled to it, and is entitled to exercise his stewardship as he sees fit, until such a time as the King returns.
Could we call him a tyrant if he was democratically elected?
He could easily be called a tyrant if he had 'taken' control, by use of economic or military power, overthrowing another ruler or government, but if his title is hereditary surely he is allowed to rule as he sees fit?
Or do you mean a tyrant in that he is not keen on the idea of relinquishing his stewardship on the emergence of Aragorn? For myself, I would have said that he willingly gives up his claim in any case, and spares Aragorn the trouble of removing a tyrant, so could he be said to have been a tyrant anyway? Or am I running this idea round in circles?