I believe the point of raising the issue of cocaine intake is that it is known that continual ingestion of the drug can cause paranoia, and lead to suspisious behaviour. This may add legal weight to the argument that the victim's erratic behaviour added to the (mistaken) belief that he as a suicide bomber.
Does it justify the shooting? Obviously not, but when you have to try and defend the indefensible, you'll clutch at any straws, especially if they have traces of illegal drugs on them!