Jack (friend's friend (metafriend?)):

Saying that they refused to serve Eru is presumptory. All we really have even semi-credible evidence of is that they refused to serve Aule, who we are supposed to believe is in turn a servant of Manwe, who supposedly has the sanction of Eru; a rather specious chain of command to take at face value given the obviously pro-Manwe/Vanyar slant of the extant texts which are, remember, canonically in-universe texts and thus subject to bias.

Me:

Eru being a bit of an absentee demiurge, one could actually argue based on the Ainulindalë that there was in fact no rebellion against Eru - in fact since Eru created Eä based on the titular metamusical corpus as a sort of interpretive dance on a universal scale, and Eru himself expressed no particular preference between what we can assume to be Manwë's composition and Melkor's, I would argue that in fact Melkor cooperating in the other Valar's vision of Arda would be rebellion against Eru.