Originally Posted by
Lylandra
True. To be a Paladin of the Silver Hand and wield the Light in their ways, you have to be anointed and accepted by the Order of the Silver Hand and be truely convicted in the course of the Light. Even if this means that - later on - you'll do horrible things because you think they are the right things to do. For a milder example than the Scarlets, Tirion was exiled from the Order because he was showing mercy towards an "enemy" (something that would otherwise be an example of the Teachings of the Light: Love yourself and Love everyone else so you can connect to the Universe).
Arthas has always been a weak young man, unsure about the right path. And when he finally found it, it turned out to be the road to oblivion. Arthas became a fanatic, even before obtaining Frostmourne, in his quest to destroy Mal'ganis. What he said to be the salvation of his people was in fact only a delusional quest to reassure himself to be on the right path.
Bolvar Fordragon on the other hand has been a devoted, experienced and humble Paladin for years. He educated Anduin when his father went missing (and when Varian became enthralled by Lady Prestor before) and he kept Stormwind going. We don't know much about him, but I guess he was a gentle "father" and a loyal friend who risked his life many times for his people (unlike Arthas who ultimately sacrificed everything for himself). His belief and strength of character were so great that even Lich King Arthas, who successfully corrupted Saurfang Jr. (not an easy task!), could not break Bolvar.
And I guess his spirit is still unbroken, now that he is the Lich King himself. He might be bitter and resent his fate, but he knows that it is his duty, his final sacrifice. And I don't know whether Tirion would have been able to do such a damn fine job as Bolvar does.