I think Emil kinda idolized Szayel for most of his youth, and sought his affection in the form of approval. Though even as a kid he was inquisitive and questioned everything, he never really seemed to doubt in Szayel.
It made his defeat at Mayuri’s hands incredibly devastating for young Emil. I think his admiration turned to disdain, something that Emil coldly disregards as a worldview that is wrong. And there is no use for such a thing, not when he is seeking the truth of it all.
When he battles Ichika and Kazui, he expresses his disinterest in avenging Szayel, much to the contrast of Ichika desperately wanting to find her father.
When he encounters Mayuri, Mayuri tries to bait him by asking if he is there to save Szayel, to which Emil responds that the battle they had was a battle of ideals, and Szayel’s ideals ended up proving to be false; he has no interest in defending that which is provably false; even if it entails defending someone who he cares about.