But remember we don't know for sure whether Luke had other knives in his possession and the murder weapon has never been found.
Neither has the murderer if you believe in innocence. There have been a number of knife finds in the locality none has been shown to be the weapon. When did he hide the weapon before or after he ran home to burn his clothes? The absence of evidence is not evidence against Luke. What happened to the gun and the balaclava? Did he have an offshore bank account to hide his drug money? You are asserting something without a scrap of evidence.