The World Eaters and the Emperor's Children and the Thousand Sons, etc etc etc were Chaos Marines long before any thought of fleshing them out pre-Heresy existed. If you do not have a soul it means that even though you are attractive and charming and work intimately with your true love, you will never consummate or even express your love, in the example of Bequin. Both of them are effectively robots - self contained individuals who despite their abilities to perform work - рабо́та - do not have their own animus - their own souls. The famous untouchables are Cain's sidekick and Bequin. That is what the setting is for, it is like a courageous character actually being a lion. The Necrontyr were effectively robots before they were actually robots. Well, the World Eaters were nut s about bloodshed and the Thousand Sons thought they were smarter and more powerful than everyone else and the Emperor's children were narcissists before any of them became formally 'chaotic.' Of course you can say oh no, before they were robots they were Necrontyr, and human untouchables aren't robots so you can be soulless without being a robot. That is the necrons' deal, they are robots.
