So first, how on earth did a peaceful, agragarian society produce a monstrous, killing machine like Szeth?
Second, Szeth is a surgebinder. How did he become one in a part of the world where the highstorms don't affect much, and stone and stormlight is sacred?
Third, is Szeth merely bound by personal choice to the oathstone, or is he forced to obey the holder of it?
I have a theory on this one. Szeth had a spren like Kaladin, but in turn, it was bound to the oathstone. So, Szeth made an oath that he would obey the bearer of the stone, and his honorspren binds him to it. If he breaks that promise, he loses his bond to the spren, and is no longer a surgebinder.
Fourth, Szeth was fairly certain that if he died, the shin stone shamans would be able to recover his shardblade. How could they, if it is really hard to defeat a shardbearer? what crazy powers would they have to overcome that?