There are many reasons that various SA realms could want to attack Aurvandil. None of it has anything to do with them being "heathen".
1) Aurvandil is the last of the self-proclaimed "League of Free Nations" that declared war on Sanguis Astroism in general, and Astrum specifically, in an effort to destroy it. (That's an approximate description, but close enough.) This was spear-headed by Averoth and Caerwyn, with moral support from Madina. All three of those are dead. It turns out that everyone forgot about Aurvandil's participation in this, probably because they mostly kept their mouth shut, and were not publicly vocal about it. It was forgotten about until various new nobles joined SA, and brought it back to the forefront.
2) Mendicant declared that he wanted to "destroy religion", and that he views gods as nothing more than a pale imitation of monarchs. Aurvandil follows "monarchism", which is not a worship of monarchs, but a view that all you need is a monarch, and that gods are worthless/useless/powerless/etc. This is a paraphrasing, and Mendicant says that he supposedly never meant that he wanted to destroy all religion. Nevertheless, he said it. It is easily interpretable as "I'm better than any mere gods." Just about every religion on the island took this as a direct threat to them, and referred to it as Aurvandil's war on religion.
3) Aurvandil is sheltering Orthodox Astroism, a heretical Astroist faith led by there heretical Allison Kabrinski. Aurvandil even helped her establish her heretical faith by giving her a lordship from which to found the religion.
4) Falkirk is a buffer realm between the Lurias and Aurvandil. Having Falkirk wiped out frees up the Luria, Fissoa, and D'Hara to join the war on Aurvandil. In a way, Falkirk is nothing more than a pawn to everyone concerned. (Except perhaps to Falkirk themselves. 
) Aurvandil wants it to survive as a continued distraction and buffer against the Lurias. (Although Aurvandil will vehemently deny this, that is the very function it performs, intended or not.) Everyone else wants it dead so that the Lurias and Fissoa can get on with the true business of wiping out Aurvandil. That and damn near everyone involved finds them to be extremely rude.
5) Falkirk and Aurvandil are inhabited by a lot of nobles that were in Thulsoma and Averoth, both of whom defied and attacked Sanguis Astroism, and Astroist realms. They are vocal opponents of Sanguis Astroism in general.
I'm sure there are probably a few other reasons, but those are the ones off the top of my head.
I hope you can see that in order to figure out why things are happening, you need to look beyond the immediate present. You need to look back into the past and learn the history of the various realms involved. And then you need to look at the effects of each of these intermediate situations, and figure out what this will accomplishing the global picture. Dwilight is a huge island with lots of realms. They are all interwoven to a great extent. Now that almost all the rogue lands are taken, and the realms are all bordering each other, politics gets pretty hectic and tangled. Nothing is as simple as it seems.
This is an excellent summary.
