You are missing the point. We know that drugs cause people to make stupid choices. Therefore we can deduce that if a person commits a murder while under the influence of drugs, the drugs may have played a role in that behavior, especially if the person does not have a history of murdering people when he wasn't using. This is not the same thing as saying that a guy who normally wears khakis, but happens to be wearing bluejeans the afternoon he walks in on his wife and the UPS guy was in any way influenced by his choice of apparel.
Put it another way: I suspect (no I know for a fact without being told) that if we were able to remove drugs and alcohol from society, the murder rate would go down.
And you know this for a fact because you know that the murder rates during the Prohibition, and subsequently during the War On Some Drugs, were a lot lower than before or after these completely excellent experiments in social engineering? Or do you have some other fact(s) at your disposal?