I can tell you it's neither Autism nor Asperger's (which is a form of autism anyway). It's not true that people with Autistic Spectrum disorders are unable to understand emotion, nor do they lack emotion. They can be very emotional, in fact. They have difficulties understanding other people's emotional reactions (if they even notice them at all), as well as body language and the such like. They come across as having little or no empathy, but this is due to their inability to read other people. A person with Asperger's tends to make up for this as they get older by using their highly developed reasoning/logic skills to work out how other people are feeling, and why they react in a certain way. What normal people have through instinct, people with ASD learn through using their intellect. Adults with Asperger's are quite able to fake it to fit in. Trust me on this. Normally, those who are severely affected can't do it, though. It's a spectrum, after all. Dexter likes to follow rules - this is an autistic trait. However, having certain traits doesn't make you an autist. You need to fit the triad of impairments to be diagnosed as having an ASD. Everybody on the planet has some autistic traits. One more thing to note is that Autism is a neurological disorder...the brain is neurologically different to that of a neurologically typical individual. If Dexter is an autist, he would have been an autist as a child too - even before the death of his mother. However, Dexter is depicted as being a normal child up until the death of his mother.
Dexter is a sociopath. His sociopathic tendencies have come about due to suffering an incredibly psychologically damaging trauma in early childhood. The thing to remember about any psychological disorder is that there is no such thing as a "text book case". All people are different, and exhibit different symptoms. Just because Dexter doesn't show all the classic signs of being a sociopath, doesn't mean he isn't one. He shows enough signs for a diagnosis. As for the blood fixation - that would also be down to the trauma suffered in early childhood...remember, he was locked in a container swimming in his mother's blood for quite a while.
(I speak from experience on the Autism thing, as I have a son with Asperger's and have Asperger's myself. I'm also heavily involved with the NAS.)