Was Vallano, Szeth’s grandfather, also a Truthless? And if not, what did he do to disgrace the Shin?
No, Vallano was not Truthless.
Szeth was a very respected member of his society, once. There are clues to what happened in his story, but you won’t hear it in full until he gets his book. (Which will include his flashbacks.)
If you are viewing this on github.io, you can see that this site is open source. Please do not try to improve this page. It is auto-generated by a python script. If you have suggestions for improvements, please start a discussion on the github repo or the Discord.