I don't think Raogrimm did it just because he was heartbroken. That was just the tipping point. We witnessed his disgust with the Humes unwillingness to even try to understand those around them. He witnessed, first hand, the oppressive hands of racism. This is without mentioning the 200 years of oppressed memories he carried with him. The scars run significantly deeper than mere heartbreak.
Regarding his personality, I'd say his lack of personality is what was most striking. He became emotionally dead. Surprisingly calm despite the circumstances. Oddly focused. Possibly a metaphor for how a small moment in our lives could change the course of our psychological history if we just happen to not have our moral / emotional defenses bolstered. With a savage, ravenous, brute of a killer you might be able to take advantage of a lapse in his judgement. No such flaw was overtly present in Raogrimm / Shadow Lord. His hatred was calculated.
His motives were driven by a reasonable disgust and hatred for the sins of the "enlightened" peoples of Vana'diel. With his entire race being treated as just one step above the Beastmen horde. One thing I never really bought though was why he didn't make his first order of business the complete eradication of the Anticans. If racial motivations were any part of his psychological profile I'd think that would be the very first thing to occur. If not, they should've explained why.
While I'm for the Shadow Lord as the "better" villain. I don't think I can say that Van Darnus is not a good villain. He's well written (considering the dev's limitations) and certainly aesthetically well designed. I think he wins in that category. He's a villain I would love to see more fleshed out. I don't think we had enough time with him. Hopefully we'll get to see more of him.
You could blend out a lot of the details of the shadow lord because he's so dark but Van Darnus' armor is so ornate and intricate. A very interesting character indeed. As per motivations though I still feel Raogrimm trumps Darnus in that respect.
Gaius Van Baelsar on the other hand... He has the aesthetic appeal of Darnus but a cold and haunting rationality. He'll get your own nation to implode internally via subversion. He can explain why you are inferior without having to kill you. He'll use you to kill yourself, submit to him, or die in a futile struggle against him. He holds all the cards and he doesn't care about inheritance.
The only thorn in his side is his own unpredictable, overzealous, sprat of a peer Van Darnus (who was probably handed his position by bloodright). If Van Darnus isn't dead, I foresee Gaius fulfilling Heirsbane's namesake yet again.
If the empire were ruled by Van Baelsar, we'd be rolling out the red carpets for the Empire rather than causing needless casualties. In fact, Darnus' pet Meteor project may be all in vain. Without Meteor there'd be no reason for the Circle of Knowing to do what they're going to do. Our only chance against the empire would be small pockets of Ala Mighan refugees and other rebels against a military force the likes of which have never been seen. While meteor was supposed to be a deathblow, perhaps the slow poison and dread caused by an eventual occupation would have broken the spirits of the Eorzeans more thoroughly, and the people of Eorzea would've been much easier controlled rather than being galvanized to fight for their freedom as a unified force.