Gee, lurking around has landed me in a strange spot. Alrighty, my turn for academically unqualified opinions.

[@bluetommy2]. Making a sociopath/psychopath into a developing character that is relatable is difficult if you're going down the route of raw sympathy. With people closer to average, you can have people like them because they're [i]nice[/i] or because they have a morality you agree with. You have to diversify a bit more when the character doesn't have these things. Here's a couple of routes you can do down, though they are not the only routes by any stretch of the imagination.

To make things easy, I'm going to make the assumption that your character isn't a complete nihilist. This means that the detail you provide about them disregarding [i]everything[/i] may not be completely true. If you ARE going down the route of a complete nihilist, you have more challenges with your character than their clinical diagnoses.

So, with them not being a complete nihilist, that means that they have something to lose. Probably. Just because they might go forward at other people's expense, it doesn't mean that they aren't allowed to value something. Put that valued thing under threat and they might have to learn some lessons protecting it.

As opposed to protecting something they already have, another starting point might lie in achieving something they don't have. This could be anything, but they still have to value it. If you make the path to that goal laid with obstacles to overcome that might cause some lessons to be learnt, you might just have some character development there as well.

In both cases, you can't change the fact that they think and act differently to most people. The thing is, even if others might find a socio/psycho abhorrent, it brings them down to a human level to still value certain things, whatever they might be. Phoenix cited Sherlock Holmes, he's got mystery solving and justice to value. Vilage linked American Psycho and the main character there, Patrick Bateman, has his status and pride to lose. I'm oversimplifying those examples, but you get the idea.

Take heed of what everyone's said, do some experimenting and practice, do a little research perhaps, and then look back over what you've done later. My word isn't gospel and you might find a completely different way of doing what you want.

P.S. Consider having a different character there to reel in the more boundary-crossing behaviours of your character. Even if it's just a foil, it helps you as the author keep the socio/psycho down to earth.

In other matters:

[@Lady Amalthea]. I can back up the opinions expressed by others with regards to Asperger Syndrome. I've had a few friends with that condition. I also highly recommend [url=https://en.wikipedia.org/wiki/The_Curious_Incident_of_the_Dog_in_the_Night-Time]this story.[/url] It's a decent novel and great reading for getting in the head of someone on the autism spectrum. They don't specify Asperger Syndrome, but it has a lot of similarities. Granted, take Morose's grain of salt; they're still unique humans and the main character in the above novel is only one person.