Lilith
Pre-Eve mythology
Jewish folklore tradition: Lilith was Adam's first wife, made from earth like him (not his rib). She left Eden after refusing to be subordinate. Rossetti uses the parenthetical to mark this as alternative scripture.
Eternal youth trick
She doesn't age while everything else does—vampire logic before Dracula. The comma splice makes 'young' and 'old' crash together in one line.
Fatal flowers
Rose = erotic love, poppy = opium/sleep/death. Victorian flower language stacking seduction and oblivion together.
The kill shot
That 'strangling golden hair'—singular. One hair is all it takes. The whole sonnet's seduction compressed into a garrote.