To prove a generalization false, "drugs do not cause violence", you just need one counter-example. I've found one example where the drugs did cause the (attempted) violence so the generalization must be false.
You're right that if the claim is "drugs are never, ever the cause of any violence" then you just need one counter-example.
But the claim is actually "statistically, violence caused by drugs does not occur at a rate detectably above the rate of violence in general" which means something like that the number of violent acts with drugs as a cause is extremely low, or that violent acts caused by drugs would still have happened just with a different cause if it were not for the drugs.
The way to invalidate the poster's original claim is to show evidence that the violent behavior in that specific instance (or at least a comparable instance) was in no way related to the perpetrator's drug-influenced physiological state. I believe that would be difficult to demonstrate.