In cosmetic doses, Botox is extremely safe. Yes, deaths have occurred in the past but were mostly related to large doses used to treat huge masses of muscle eg in spastic muscle conditions in children. The LD50 (dose required to kill half of patients injected) is 'estimated' (no one has actually done the studies) to be in the thousands of units whilst you probably get less than 50 in most facial injections.
There is probably a lot to come out further in this case, but I'm sure just getting some Botox to the face was not what killed this patient.
, actually Botox is one of the few instances where the mechanism of action is pretty well known and is pretty specific in what it does. Compare it to say Aspirin which can have a myriad of unwanted reactions particularly in children.