I read your post this morning before I went to work, and it sparked a train of thought that I couldn't get out of my head all day. So I figured I'd set up a log-in to respond with a few musings. Don't have a lot of experience participating in this sort of thing and I hope this is ok for this thread...
I see a lot of parallels between Szeth and my other favorite Cosmere inhabitant, TenSoon. I'll limit the scope of my theory to Sezth and leave you to fill in the Mistborn parts. I don't know that I've seen any hints as to Szeth's place in society before he became the world's most affordable contract killer, but Stone Shaman seems more likely than expert turnip farmer. That being said I think the 'Truth' in 'Truthless' almost certainly refers to some kind of religious belief. His betrayal was not something tangible like a theft or a murder. A respected figure publicly endorsing a heretical belief (the voidbringers are coming, the oathpact has been broken, ect.) could certainly be seen as a betrayal of a whole people. He is Truthless by virtue of rejecting a Truth of his people and thus cursed to wander the land bound to the whims of a (maybe?) arch villain... Yet Sezth is in fact more 'pure' in his beliefs and his honor forces him to act to save his people.
This would fit with some of the common themes in these books. We've got a good five years or so to speculate on the details, but underlying ideas about faith versus a corrupt or mistaken establishment seem to be coming from similar place.