He is a shadow sun -"shadow" in the archetypical sense- he is all the downsides of what the bond of love/heart, is and can be. He draws people to him with false charity, guidance, and nurturing only to throw them away for his own benefit when needed. It and its powers are conditional, fickle, hierarchical, and ultimately exploitative. But Yhwach's "love" is not a healthy bond. Yhwach can fulfil the function of a Sun, just like the Soul King does, like Ichigo and Jugram(And Uryuu?) implicitly can, and just like Aizen tried and failed to do. you'd think that should be the other way around. Yhwach however isn't himself a symbolic anti-sun the way Aizen was, he's an alternate sun Although ironically it was Aizen who wanted to be god, and Yhwach who wants to undo the world. He's an anti-sun, the absence of sun, of light, of warmth, of love, of heart, and of connection. Aizen was not a Sun-like character(and although not in direct contrast to the sun in text, he is of course very explicitly compared to the moon repeatedly), but he tried to put himself in the position of one, and he failed to replicate that role's effects. But his army was fraught with in-fighting, and he himself didn't really value them, even as he forced that organizational structure on them. But there is a little more to that, because when he defected from Soul Society and vowed to become a god, where did he go but a place of darkness and night: a place opposite Soul Society and furthest from the Light and from God/The Soul King? And more than that, he built Los Noches under the light of a false, illusory sun and united under its roof an army of beings defined by the removal of their hearts. As an interesting parallel to this, Ichigo kind of hastily and not too convincingly gives us the idea that Aizen wound up the way he was because his power alienated him from others, and that what he lacked was love and understanding and companionship: human connections.