The Stepteria has three principle things going on…set aside the whole fighting a dragon/monster scenario. Really that has little to do with it. Dragon is what nature designs its nature to be. Being harmful to people is a side effect of its massive territorial nature in western myth (as would be the consuming of livestock). Delphyne is neither a god nor a man. Typhon, whom she reared at the request of Hera, was evil, because he was not a dragon or a creature of such innate nature. As the child of god, while the way he reared may have ruined him in many ways instilling in him a destructive lifestyle, he was of a higher state. He was of the divine stock, whereas Delphyne was a creature of the earthen stock. The Erinyes are not exactly pleasant and are considered pretty baneful but they are not considered evil as the do as their nature dictates that their role in the cosmos is. Dragons do as their natures dictate that they do. So set Delphyne more or less aside in the specifics of being a monster (something which distinguishes her from the other Python who looted pilgrims to Delphi). Apollon slaying her was not personal, but a matter of just what he did…as was the purpose of decaying her on the ground.
This leads me to the first point. Delphyne in many ways represents the purpose of Apollon as Pytho, the decayer. He frees the spirit from the miasma that has attached to it simply from life. He frees the soul from the burdens of its life and form. The most expedient way to do this was by burning (hence we see early practices too of funeral pyres being used). However painful it is for us, it is a liberation. And how liberating it is for us, it still instills grief among those who are left behind. Why would the residents of Delphi have funerary rites for Delphyne if not to appease her and a sacred relationship with her that they benefit from. Her bones and flesh literally becomes the sacred precinct. She is transformed into a guardian spirit of the precinct by all of this. But all it carries certain ramifications….and his own purifications for having to deal intimately with it is part of his cycle. I would say that Delphyne allegorically takes the place of every innocent life he slays out of his natural role.
This is the second point. Apollon exacts nature’s laws in which Thanatos operates, in that which is born and lives must die, that which is miasmatic must be expunged for the welfare of all. Stepteria ought to make us deal with very harsh realities about our own natures and that we have our own “impure” inclinations that are just part of being living beings. And that at the end of our days we will have to lay down to rot in order to be freed and progress. As such this cycle with Delphyne purifies Apollon by the river Peneios and by Daphne for his duties in nature. There is a reason why Apollon is associated with so many purifying agents…..it is often because he himself is need of their use prior to anyone else having their use available to them.
Lastly, it is a recognition of transformation and change. It is unyielding and always eventual. There is nothing evil in the act of destruction but it is a necessity with its own consequences within nature. We cannot find against it, but ought to celebrate it even as we mourn for it. Change is hard, but it is a blessing even if it doesn’t appear so. That Delphyne’s bones and ashes were used to make the foundation of the sacred precinct of Delphi we can not a punishment (and while Apollon did punish Telphousa in myth, there is really no indication that his battle with Delphyne was a punishment but rather the movement of forces.
By celebrating the Stepteria, we see that it is aptly named….the crowning festival. This may seem at odds with the rather downer mythic component of exile. In reality it stresses victory. victory of the god yes, but also a kind of spiritual victory. We ask that the god crown us, that the god slay us, destroy us, and remake us by cleansing us. The boughs that were gathered likely to crown heroes from the Pythian games probably has the same emphasis. The proof of worth, the remaking of the self to be crowned heroically to dwell among the blessed. Stepteria is a most holy sacred festival of our lord even as we grieve and purify as we await to celebrate his return. We identify with Apollon and with the serpent..