Long before they discovered the Books of Laroth and became Shades the Sidar were a group of people who worshiped Arawn. He of course never cared for their worship, so while many Sidar still revere him to some degree or another they generally don't bother worshiping him anymore. Arawn has never done them a favor of any sort, but his Archangel Gyra did once intervene on her own to save them from total destruction at the hands of her twin brother Basium.
Death is not so much the fate decreed for all living things as it is the manifestation of surrender. It is the opposite of Life, which in FfH terms actually means perseverance and the willingness to sacrifice that which one holds dear for the sake of something greater. Death is about rest and apathy, the willingness to stand by and do nothing while the world passes you by, to stay in one's comfort zone even when one probably should not. Life is active, while Death is passive. Life forces its will on the world, while Death leaves others alone.
The Sidar kind of immortality is actually more like dying very slowly. Waning causes one to lose all passion, everything that makes one feel alive. It is clinging to that one holds dear at the expense of many greater things. It soon leads to apathy, so shades carry on their old occupations purely out of habit and after centuries of meaningless activities many shades give up on that and just sit still staring off into the distance as they fade almost into nothingness. Many Sidar eventually grow tired of life and thus commit suicide.
The Sidar know that the Books of Laroth came from the Netherworld, but they don't know about Laroth. They may think that the books were written by ancient priests of Arawn, rather than by the archmage who is currently leading a rebellion in hope of destroying Arawn and usurping his goddood. They have no idea that their souls are being used to force weapons for his army and power the rituals of his eventual ascension.
The Waning ritual is not actually death magic, but Spirit magic (maybe with a little dimensional thrown in). Death magic is actual UNdeath magic, as it works by preventing the dead from finding rest. The Sidar may be fine with postponing death for themselves, but they abhor the idea of keeping a soul in this world against its will. They oppose death magic more than anyone in Erebus.
