Someone mentioned SIDS upthread as being an alternative cause of death, but isn't one of the theories of why SIDS occurs that the child is unable to rowse themselves? From what I recall from some articles I read, the child isn't getting adequate oxygen, but rather than repositioning themselves they end up gradually running out of oxygen.
In which case administering a sedative could be contributive.
I remember reading that deep sleep for a very young baby should actually be discouraged and that them waking every few hours was normal and protective.
Really sad that these parents were let down so badly by someone who presented themselves as trustworthy (assuming it was the nanny who administered it). I think people should reserve their judgement on why they outsourced the night care - we don't know what the mother was going through postpartum, what sort of support she had, or how well the baby was sleeping during the day and night. If the mother was massively sleep deprived and the father unreliable/often working away then that could lead to not being able to care for the child safely, so this may have been the most appropriate choice at the time.