I believe that Zeus has free will but can’t officially choose Nami yet out of fear. But if Zeus can exercise his free will to overcome his fear of Big Mom by choosing Nami, he will become his own man and live life on his own terms whether Big Mom lives, dies, or gets mad.
Among other things Zeus’ loyalty to Big Mom seems to be rooted in his fear of her. By contrast, Zeus’ temporary stay with Nami seem to have largely been a consequence of his vice.
However, Zeus and Nami’s relationship seems to have evolved to the point where Nami considers Zeus her friend rather than her slave. And Zeus seemed to have developed an emotional attachment to Nami, one motivated by something other than gluttony.
While Zeus tried to kill Nami, he tried to do so under duress, out of fear of being erased. I believe that this moment will eat at Zeus’ conscience as the arc progresses, making him question his allegiance to, and his life under Big Mom. He will find his way back to Nami.
I think in addition to feeling horrible about trying to kill Nami, Zeus will officially betray Big Mom and choose Nami because of her virtues — Her strength of will noted by Kinemon, her kind and compassionate spirit revealed by her actions on Punk Hazard and Zou, and perhaps her “queenly nature” per Oda.
Remember, if Zeus overcomes his fear and leaves Big Mom for Nami, it would mirror Jinbe’s fearless action in Whole Cake Island. In other words, the precedent for someone to find freedom by fearlessly standing up to Big Mom has already been set in Whole Cake Island.
I think Zeus choosing Nami over Big Mom will manifest itself by way of Zeus allowing Nami to ride him, perhaps into battle. Now that would be epic.
Finally, Zeus choosing a life with Nami, not as a gluttonous slave but a genuine friend & helper, would not only free him, it would also be on brand with some of the major themes of the arc — among which are freedom, identity & betrayal.
*Theory by @KaizokuWave56 (Lelouche85)