Factual error: Immediately after Eddie surfaces following his dive, he rests with his head well above the water. Had he actually been in water deep enough for the dive, he would have floated with his chin about even with the surface.
Factual error: Eddie claims it takes 30s for the drug to take effect. Tablets/capsules do not work within 30 seconds after ingestion. At most, 15mins is the time a drug can be absorbed into the body. This is also noted in the movie reviews.
Factual error: During the shootout in the apartment, the blinded henchman fires his handgun 24 times before reloading (and about the same afterwards). Only small-calibre pistols would have that capacity, and the gun he's using isn't one (either to look at and from the bullet holes it causes).
Answer: Although there is no definitive proof, I believe the killer to be Atwood's henchman. During the trip scene we see him following Eddie and the Blonde to their room and although it comes off as an illusion there's no reason it cannot be real. This alone is not enough to say for certain but the main reason I point to the henchman is because of how the story plays out following the murder. Eddie is by an eyewitness (probably someone working for Atwood if not the actual killer) who tells the police about him and as a result he is called into questioning. Because of the inquiry Eddie hires "the best lawyer in the city" who as we know is under Atwood's thumb. It is during this line of questioning the lawyer is able to go into Eddie's jacket and steal his NZT. None of these things would have happened had the Blonde never been killed.
dream3ater