One of the things I love about Dexter is the complexity and duality of his nature – creating a paradoxical character that is both obsessed and repulsed simultaneously with blood.His relationship with blood, having dealt with it since an early age - to have sat in a pool of his mother’s blood for a long length of time obviously gave him his need to understand it fully, and deal with the inner demons/ trauma created from the corruption of the innocent child he was.
In order to do this he does two things, firstly using forensic science to analyse it fully in its biological form, by beginning with samples and testing the nature of blood, what it is, where it comes from, etc also by studying blood trails and splatter forms to further understand how blood reacts when cut from the body.
This is where we see his need to make blood “clean” by storing, labelling, numbering and photographing it’s varying forms Dexter understand of the relationship of blood with the body.
Secondly he furthers his understanding by expanding from the first method to the actual method of removing it from the body from stabbing a dead rabbit to see how it bleeds, to the full routine that he perfects of his kills – he still keeps a method and having everything as clean and hygienic as possible he removes the full gore of the situation. Yet he always succumbs to satisfying an inner need that has an insasable appetite for blood and submersing himself in it fully, this is where we see the root of his true obsession and almost carnal desire to recreate his childhood situation in a controlled, “safe” and clean environment.
To Dexter blood has become all he knows – and part of what he is it’s his nature he has become the monster that protects the child within.
Final note of interest; blood is often seen as a sexual and powerful thing (think vampires) and there is a connection to the two that is felt by Dexter too - since he feels his true nature would be revealed if he "opened" himself to a woman fully, and although he avoids it to a certain degree he shows that he has a hidden sexual lust also.