She did it for revenge on those who kill Muslims, shouting "Allah hu ackbar" and threatened to detonate herself on a busy Moscow street
An interview with the nanny in her jail cell
Below, a news clip from Russian TV which explains the events.
The narrative generating machine seems not to have started entirely in Russian media but look for attempts to dismiss this religiously and politically motivated act of terrorism as a mental health issue or an 'anti-Islamic act' and almost certainly as a criminal-justice issue, despite the fact that committing an horrific act for a political reason against a soft target such as a child in your care, is a perfect description of terrorism.
One might add that it would take some research to find acts of brutality against soft targets done in the name of any other religion in modern times, and for reasons of religious manifest destiny.
One might be tempted to question whether nationality is more than just an accident of geography or needs to contain a shared set of values. Perhaps that is why there is such a massive media effort to deflect and reframe events in the narratives of crime or mental health. To avoid any discussion of real motives and Islamic values.