If I were to play armchair detective, I would surmise that Harlow's accomplice was laying in wait, unbeknownst to Bryan. Since Bryan was, or appeared to be, familiar with Harlow, his guard would be down. He would have been attacked and had his throat slit from behind, therefore no defensive wounds on his forearms. What the reports coming out fail to mention is if the charges of corpse abuse are from the burning of the house and, sub-sequently, Bryan's body, or of being stabbed 28 times post-mortem.
Having his throat cut from behind also wouldn't leave trace evidence, like blood, on the attacker.
Again, just a total and complete assumption. Do not take it as gospel or fact.
R.