I see several issues here, but I think it is extremely important that no one leaves thinking Valium is the most powerful muscle relaxant available. Valium is a centrally acting muscle relaxant, among others. It is very difficult to kill someone with an overdose, mostly interactions with other drugs.
The best muscle relaxant / antispasmodic depends on the patient and the situation. Valium is a safe indication from someone not dealing with those medications, as a temporary measure.
All benzodiazepines (including valium - diazepam) create dependence.