Yeah, an Anders romance turns DA2 from something of a tragedy, in the traditional thematic sense of the term, into a full blown one.
And yes, although Sebastian does somewhat diagnose Anders correctly (I wouldn't say selfishness was his problem....the problem is a very real and overpowering desire to see mages freed, coupled with merging with a sprit with no real concept of time, a lack of alternative options, unwillingness to compromise from his goal and an ethics of "the end justifies the means") let us remember that Sebastian, as a devout lay brother and potential romance for Hawke, is not exactly a disinterested party here.
And yes...that threat. I don't want to spoil DA:I for anyone, but let's just say it is referenced in the game as to what becomes of that.
I've said this in other places, and I can understand why Anders did what he did, without condoning it. The Mage-Templar struggle was a war, though no-one was willing to admit it. The Templars refused the notion of compromise....and so did many of the mages, turning to radical libertarian factions and blood magic to win their their freedom (and if there are many mages who definitely shouldn't be allowed complete freedom, it's the ones with mind-control powers).
The situation was at an impasse, but could likely drag on for decades more, with less radical Templars and more Aequitarian mages attempting to find solutions...ultimately failing because of the direction the Circles and the Order were taking, but prolonging the inevitable conflict, and making the situation worse for everyone.
Anders' solution, such as it was, was to make it impossible for the moderates to negotiate, to "remove the possibility of compromise, because there can be no compromise" in his words. In a strategic sense, it's actually quite a brilliant move. It completely alters the terrain of the conflict to the terms he wanted, the outright warfare between mages and templars, where the conflict could finally be settled in a decisive fashion.
Of course, it's also a flawed view, because nothing is ever settled in a decisive fashion in politics, and the question of mages is of course a highly political one in Thedas. Even if the mages "win", the Chantry view is too deeply entrenched to be replaced simply through military victory. And the other problem is of course magical war weakens the Fade, which based on the last two games is not so much of a strict wall between realities as an extremely thin and sometimes virtually nonexistant barrier.
And it's also flawed because it requires the death of thousands of innocent people (maybe. What was the death toll from the Chantry exploding? It didn't look good, that's for sure).
But then, that's also the part of the brilliance of Anders, as a character. He encapsulates everything that is wrong about the Templar oppression of mages, with everything that is wrong about groups and indviduals who use extreme and bloody methods to oppose their oppressors. No reasonable person can deny the mages are treated very badly, at the least in Kirkwall...but equally, nor easonable person can deny Anders response to that was also a terrible crime. The moral dilemma is expressed extremely well, this thread and the hundreds of others like it being excellent proof of the case.