@ all: that's really helpful, thanks. Upvote to the first one (the quote feature doesn't work on my computer, sorry) who mentioned Sadeas loves his wife, because I never thought of that, and yes, I agree that is important to consider when comparing to Straff.
I always thought Tonk Fah was a little messed up in the head, but maybe not enough to say he isn't evil?
Kaellok, if I am to understand you correctly, are you saying that an attempt at evil actions (which is like intent plus action), whether successful or not, is what makes a person evil?
What you say about Szeth reminds me a lot of a definition of second-degree murder. If you do something to somebody on purpose, knowing full well that it might kill them, and then it does kill them, you are guilty of second-degree murder. If there is also intent to kill, then it is first-degree. Either way it is at least second-degree. Szeth is a murderer in the first degree, but his attempted murder of Adolin "on his own time" should be placed in a more severe category than his other murders, (because he didn't even try to make an excuse there?). Perhaps if he hadn't tried to kill Adolin, Szeth wouldn't be as bad? I don't think Szeth would have murdered "on his own time" earlier in his killing career (though maybe when we find out why he became a Truthless we'll know more)
The debate on who of Szeth or Taravangian is more evil is interesting. I used to think Taravangian was worse, hands down, but now I am not so sure. After Szeth's book, perhaps all of us will be more sympathetic of Szeth.