I agree, my bet is that the victims were like most suburbanites buying arguably commercial amounts of drugs is that they just wanted to make their purchase and leave as quickly as possible. The drugs they did not use themselves, were probably destined to be given away (well to select females) or sold user to user level.
The victims probably had no aspirations of becoming rivals to the seller, ripping off the seller etc. Rather, they just wanted to enter the criminal world briefly, and then return to the other world asap. The fact that they were buying from somebody known to be creepy and aggressive could indicate that they were not deeply involved in the drug trade and had no other alternative sources.
Likewise, my guess is that the other motive could be pretty fuzzy: A not too bright cartel wanna'be, or as other posters have mentioned, an evil person predisposed to commit murders who then finds a convenient setting though his drug dealing.