Both Fred and Serena Waterford brutalized and tortured June Osborne for three seasons on The Handmaid's Tale, but June's hatred runs deeper for Serena than the Gilead Commander. With June on the run after orchestrating the "Angel Flight" and Serena awaiting her trial in Canda, the two women haven't been face-to-face since season 3, but with June finally escaping to Toronto, it doesn't take long for June to reunite with one of her former captors.
During Handmaid's Tale season 4, episode 7, "Home," as a traumatized June attempts to adapt to her newfound freedom while being plagued by brutal flashbacks; she learns that Serena is pregnant with Commander Waterford's baby. This news sends June over the edge, culminating in a confrontation where the women have two very different agendas: Serena believes God has brought June to her so she can make amends, and June is there to make it clear that Serena will never be worthy of redemption. The drastic change in each of their circumstances is brought full circle when June asks Serena, "Do you understand me?" These are the words Serena said to June verbatim in season 1 episode 3, "Late," a scene that began to expose the depths of Serena's cruelty. Given the opportunity to unleash her fury, why does June choose a pregnant Serena Waterford, and why does June hate her so much more than Fred?
Serena is horrible to June throughout the first two seasons, and attempts at truces are short-lived, but Nichole's birth causes a shift in the women's antagonistic and manipulative dynamic. Serena begins to see the bigger picture of how oppressive Gilead is and how dangerous for girls in particular. June encourages Serena to do the right thing and use her influence to initiate change. When their plan fails, Serena agrees to let June take Nichole out of Gilead during The Handmaid's Tale season 2 finale. This sacrifice paves the way for an alliance between the women at the start of season 3, with Serena even feeding June information about Hannah. June doesn't forget all of the horrible things Serena has done but chooses to see her as someone corrupted by Gilead, who is motivated by motherhood to fight back against the patriarchal society. In the end, Serena betrays June by attempting to get Nichole back, proving to June that Serena is entirely self-serving and her obsession with having a child - the primary reason behind her support of Gilead - has left her morally bankrupt.
Serena has always posed a more credible threat to June's children than Fred, and when June pleads with Serena in season 2 to move her to the same district as her daughter, Serena refuses. Hannah is scared of June, having been fully indoctrinated by Gilead, and June holds Serena accountable. June's hatred of Serena stems from Serena being a woman and, more importantly, a woman who extolls the importance of motherhood. However, she doesn't acknowledge, respect, or support June's desire to fulfill that role. Men in Gilead, particularly Fred, view children as currency, something they can trade for power, freedom, or goodwill, and June has never made the mistake of misjudging his motives or expecting empathy.
In Gilead, women are part of a hierarchy that breeds distrust and resentment. The compelling female relationships on The Handmaid's Tale are the ones that transcend this hierarchy, but they are also the ones that become the most embittered when the effort to do so fails. To see Serena rewarded for her behavior is a slap in the face to June. June has been denied the opportunity to be a mother both directly and indirectly by Serena, and even if Serena was at any point genuinely sorry for what she's done to June, their emotionally wrenching confrontation has left her bereft of guilt, driving Serena back to Fred. Serena and June's shared love for Nichole gave June hope that the shared experience of motherhood would bring out the best in Serena, but in the end, Serena's betrayal severed their tenuous bond for good.