I don't think they tried nearly hard enough to show the dangers of mages in DA2 (or DA:O, to be honest). In almost every case, the Blood Mage blamed the Tempars, either directly or indirectly, for their actions. And given that the Kirkwall Templars were portrayed as rapists, murderers or idiots, it's really hard not to see the mages' point.
If the Templars had been shown as reasonable, or at the very least just trying to do the best they could in a terrible situation, and the Mages had chosen blood magic and demons for reasons other than "The Templars are awful", then we might have something to talk about in regards to the factions being portrayed in DA2.
As it is, I think they finally started to get the feel in DA:I that they've wanted all along. It seems jarring mainly because they've botched it so badly until now.
But the problem is that mages are supposedly educated and intelligent, or at the very least book-smart... and still act at times as if they suddenly went retarded. I happily bash at Fiona for what happened and say that I regret not having an option to execute her for crimes against fellow mages, but let's be frank: it's more like she at some point caught an idiot ball... and then swallowed it or something, since simply holding it wouldn't derail her like that.
Though I guess it's a matter of poor delivery, really. It was intended as desperate move by the mages - a bad, stupid move born of fear, but understable. And that would be a nice element of the story, really, if not for the fact that the game failed to present the situation of mages as desperate at all! We hear that the war isn't going great for them, but we hardly see it. And we most definitely don't encounter anything that could give us a feeling of imminent defeat awaiting mages if they don't do something quickly. Just look at them: they have access to a powerful, strategically important fortress in friendly country, the common people are scared but not really hostile to them - if anything they seem to wish for the "true rebel mages" to actually move their assess from Redcliffe and do something about those mad apostates and rogue templars wrecking chaos through the Hinterlands. They perhaps don't have prospects of winning the war but they seem actually pretty safe! Just think about it - if the Templars were to actually make a move against them, that would mean attacking Redcliffe and Redcliffe Castle... pretty much an act of war against Ferelden, even if the latter didn't ally with the mages to extent of providing military assistance.
Basically, the game presented us with plenty of evidence that situation of rebel mages in Ferelden wasn't bad, much less "desperate". And then there were also the thing that allying with Tevinter would antagonize the friendly country they were actually in at the momet. Not to mention how cartoonishly evil Tevinter tends to be. Selling themselves off to Tevinter isn't even stupidity when I think about it - it's a plothole...