Good thing you didn't murder him, he might not have known that wasn't cocaine, and going to jail for that would not have been something your daughter wanted.
Why on earth were you downvoted? That seemed like a sensible comment to me.
The threat to murder was more of an expression of the mother's grief than an actual threat — in such grief and shock, humans tend to want to find someone to blame. In her right mind I'm sure the mother would never truly consider murder.