Seligman's motive - A theory for those enraged by the ending [SPOILERS]
Lars may be notorious for certain cinematic devices, but abrupt and out-of-place endings isn't typically one of them. So, the question remains for many: Why did Seligman, the cornerstone of morality throughout the film, do a complete one-eighty in the last minute or so of the film?
Let's go back to one of Joe's last chapters and stories: The interrogation of the pedophile while she was "debt collecting."
Seligman reacted very strongly and very negatively towards this story in particular. While this reaction seems obvious at first, take a step back and review it in the context of the entire epic. Seligman has remained calm and open-minded through talk of sexual assault, dangerous sexual competitions, and morally questionable liaisons of every variety and severity. So why is he suddenly so opinionated, and judgmental? Why this particular taboo?
If you watch his face as Joe justifies her actions towards the pedophile once she's finished interrogating him, it seems as though something very subtle changes in him. It's the only point in the movie where he feels almost defensive, even though he has stated that he is an asexual virgin. So what is it, exactly, that makes him react all of a sudden, after everything else he's already heard?
"This is a man who had succeeded in repressing his own desire, who had never before given into it... right up until I forced it out. He had lived a life full of denial and had never hurt a soul. I think that's laudable." ... "Sexuality is the strongest force in human beings. To be born with a forbidden sexuality must be agonizing. The pedophile who manages to get through life with the shame of his desire, while never acting on it, deserves a bloody medal."
It seems that, while Seligman was likely telling the truth about having been a virgin, he was using asexuality as a way to cope with an inner demon. Not pedophilia, necessarily, but some sort of violent, sexual urge of a forbidden nature that he struggled to suppress. Joe unwittingly "forced it out of him," by giving him an opinion that contradicted with his own self-loathing; that a man "who manages to get through life with the shame of his desire, while never acting on it" is not a detestable person, but someone of strong will.
In short (TL;DR) - it seems as if Seligman was not truly asexual, but carried some secret with him which he paralleled with the pedophile in Joe's story. Subsequently, it seems he let down the guard that he held so firmly in place his entire life, after hearing Joe rationalize and sympathize with that man, and finally caved in to his desires.
Now, does any of that justify what Seligman did? Of course not. I offer this take on the film's ending purely from my observation of that scene and the context, as possible pacification for those who were enraged by an ending that appeared to come from nowhere. It wasn't out of nowhere- just out of a very subtle and brilliantly executed parallel between Joe's story and Seligman's life.