She was only interested in keeping her throne. If that means siding with her father, who killed her husband, started a civil war, allowed slavery to be brought into the capital city, and let everyone who could end the blight perish... She's okay with that, as long as she gets her throne back.
She's a cold, ruthless aristocrat who puts her own desire for power over anything- over the memory of her husband, over her father's name, over the only people capable of saving "her" country from the darkspawn, over the good of the people she'd be ruling over.
And that's a pretty compelling reason not to like someone, in my opinion.
:EDIT: Also, as I recall (it's been a while), she acts like a ***** towards Alistair, even if he doesn't want the throne, because his blood alone makes him a threat. And then assumes his brother/sister Warden, and possible best friend or even lover, will be okay with her attitude and help her keep power.