The driving force behind Billy Butcher's affiliation with "The Boys" is his need for revenge on Homelander. For years, Butcher is led to believe that Homelander is the reason for his wife Becca's (Shantel VanSanten) death as a result of a sexual assault, although Homelander claims it was consensual.
It's not until the end of season one that Butcher discovers not only is Becca alive, she's the mother of Homelander's son. Rather than letting Butcher blow sky-high along with Madelyn, Homelander decides to take him to see Becca. Homelander isn't doing this as an act of kindness, he's doing it to rub in the fact he has a son with Butcher's wife. With a smug grin, Homelander lets Butcher see Becca before the season ends on a cliffhanger.
Now that Butcher knows the truth, season two will see how the foes' relationship changes.