While it bugs me when people declare they have certain syndromes without actual clinical evaluation (and sometimes even without it), I personally believe myself to be somewhat sociopathic ... one of the harder-to-recognize mental diseases in our world. And I think it'd be safe to say that Raito/Light is truly sociopathic.
The wonderful thing about sociopaths is that they appear to interact with the world in a completely normal way ... but their thought processes are foreign and frightening.
pretty much spoiler-free - nothing after chapter 40 for certain. Sorry if this information is ridiculously old - it was just on my mind.
Raito can use his words to devastating effect. Ridiculously persuasive, he finds it remarkably easy to convince people to see thing his way, do what he wants them to do, and gain their trust. People believe of Raito only what he wants them to believe. His natural and apparently easy way with people makes people fall neatly into his patterns of thought.
manipulative and conning
Along the same lines as charm, Raito can manipulate people into situations and force them to do things as he wishes using his superior planning abilities and conning skills. In Raito's mind, people are little more than pawns that he is destined to rule.
grandiose sense of self
Raito fully intends to rule the world, and really believes he is best qualified to do so. Need I say more?
Raito never compulsively lies as some sociopaths do, but he does lie constantly and easily with a perfectly straight face. Lies don't bother him; they are just as valid as the truth in his mind.
lack of remorse, shame, guilt
Despite the fact that Raito is killing at will and manipulating those around him to kill, Raito never feels a twinge of guilt himself. After he recovered himself from the first few deaths he caused, he went on to be a ruthless, impulsive killer. He coldly plans murders and executes them.
Raito doesn't seem to feel any of the emotions he exercises very deeply; most of his outer emotions are contrived or performed to evoke certain reactions. Inwardly, the only emotions he seems to feel with any depth are contempt for L and a conviction that he himself can do no wrong.
incapacity for love
Raito holds no one in high regard except himself, and he obviously doesn't care enough about his family to be concerned what would happen if, say, someone found his hidden Death Note and the house burned down as a result. The convincing statements he makes about killing Kira himself if something happens to his father are hard to read; is he just lying, or does he really care that much about his father that he would kill himself?
need for stimulation
Initiating arguments; sparring with L; Raito keeps himself entertained in many ways. Although he seeks to kill L, he seems to enjoy keeping one step ahead of the investigator always. He lives as close to the edge as he possibly can.
lack of empathy
Raito does not sympathize; he merely uses. The emotions of those around him are tools to be used and played with. He uses the fear he creates in the poplace to control them, and he looks forward to using that fear to rule them.
lack of realistic life plan
Raito's life plans are to rule the world. Any questions?
Raito murders people for a hobby. He also chooses to lose his memory in order to prevent himself from getting caught at his crimes. He has an easy time manuvering around people while hiding his true nature.
sociopathic tendencies not shown:
promiscuous sex life