Finished this last night. Great show, very addictive.
I could be wrong, but having checked out what others have said, I think people may missing what I think is going on with Holden. People saying he is a dick, that his relationship with his girlfriend doesn't make sense, well...maybe there is a reason?
Holden, to me, appears to have some of the similarities of the people they are interviewing. Now, this could be viewed as simply cunning and experience in the FBI and comes from his training, but I think this is where his relationship with his girlfriend brings more insight into his character.
They get along well at first, he finds what he perceives to be an equal. Eventually, the magic wears off. At one point he tells her that he just wants her to "be his girlfriend" and appreciate how awesome he his. He is looking for attention. He has a very high opinion of himself, but he can't read her feelings, outside of reading external things. Like how she is crossing her arms, or how much wine she has drank, etc. He can't put himself in her shoes, but he is very good at guessing her next move based on his knowledge of people and how they treat there external environment.
Googled this. Stages of a narcissistic relationship:
1. Idealize - "Wow you are really smart, like me!"
2. Devalue - "Why are you always upset? You just talk about you stupid classes. Just be my girlfriend and tell me how great I am"
3. Discard - "I can see you are looking to break up with me. This isn't working. OK, bye."
If you look at the people they are investigating, they say, at one point, that the serial killers lack empathy. Holden lacks empathy. Narcississits/sociopaths/psychopaths, which someone mentioned earlier, view people as objects, they cannot put themselves in other people's shoes.
So, Holden is able to discern the thoughts and motivations of the people they are interviewing, because he is like them. I think, and again I could be wrong, that Holden actually may have characteristics of narcississits/sociopaths/psychopaths, but he has yet to act on anything in his warped mind, so he seems "normal".
This is why the last scene of the last episode is key. He is starting to realize that he may be like these killers and it scares the shit out of him. They see him as a "friend", someone who understands them, one of their own.
I could go on, but I think I am rambling at this point.
Anyways, really looking forward to Season 2.