"as if it had been the Massacre of the Innocents"

Rubens' second Massacre of the Innocents
Public DomainRubens' second Massacre of the Innocents
The Massacre of the Innocents was painted twice by the Flemish artist Peter Paul Rubens (1577-1640), once in c.1611-12 and once in 1636-38. The earlier version was lost in the eighteenth century and only rediscovered in 2001; therefore Swann is probably thinking of the later one.