I have a theory that Szeth was a developing surgebinder/ Knight Radiant prior to becoming the Assassin in White. When he tried to tell others about his powers, or even that the Knight’s Radiant were returning, they refused to believe him, and renounced him Truthless.
The first evidence I noticed of this is from Words of Radiance. This is a passage from Kaladin and Szeth’s first encounter:
Szeth is horrified and enraged after hearing Kaladin claim to be a Windrunner. He cannot believe the Knight’s Radiant are returning, because if it’s true then he was wrongly named Truthless and all the killing and suffering was put upon him unjustly.
Even stronger evidence comes from Oathbringer when Szeth is confiding in Nightblood:
I think the voice he is remembering was a spren he had bonded in his youth. A spren he ultimately killed through his actions as the Assassin in White.
Does anyone else have any opinions? If he wasn't previously a surgebinder, what was the voice he was referring to? Also, I found it surprising that I couldn’t find anyone else talking about this. Feel free to let me know if there is already a post somewhere on the same topic that I missed.