How does Xenagos dying serve the overall plot of Theros. It seems like he was more a patsy for Heliod, in the sense that Xenagos was a threat that allowed Heliod to manipulate Elspeth.
Heliod wasn't the one responsible for Xenagos's plot.
What we're looking at is two separate villains whose own goals were actually opposed to each other, and the hero caught in the middle of their petty vendettas.
Pretty much. It was like in RTR, where Ral Zarek and Lazav both shared the role of titular antagonist, even though their agendas couldn't possibly be more opposed.
He is quite a promenient antagonist character throught the trilogy. Sure, not literally eponymous, but he might as well share a third of the letters of the title.
What we're looking at is two separate villains whose own goals were actually opposed to each other, and the hero caught in the middle of their petty vendettas.