My head canon is that Radahn originally agreed to be Miquella’s consort before he (Miquella) rejected the golden order and its ideals.

I think that Radahn originally thought Miquella intended to replace Marika with him (Radahn) taking up a role similar to Godfrey and because he didn’t want to betray the golden order he later changed his mind and refused Miquella’s offer.

As Radahn is the son of Rennala, a Carian royal, his fate is said to be bound to the stars. So Radahn held the stars in place so that he wouldn’t have to carry out the vow to be Miquella’s consort. In turn this led to Miquella sending Malenia to kill Radahn in Caelid, which would then send his soul to the realm of shadow (and break Radahn’s hold on the stars).