Gaia poisons Eirene to have Pullo for herself.
She makes a deathbed confession of her crime to Pullo, who does the fastest and most chilling transformation from grieving lover to cold blooded killer I've ever seen and casts her body in the Tiber.
But...remember Pullo brutally murdered Eirene's beau-a decent chap by all accounts- for the same reason.

Do we cut Titus some slack because he acted out of impulse in broad daylight and condemn Gaia for her cold premeditation?

Also Pullo killed a slave, a social inferior, while Gaia slew her mistress and superior.

Many layers in this tale. Damn I'm gonna miss it!