So yes, don't execute if you don't want bad traits. And then you have to roll that 6% twice again to get the second tier of the trait, and two more times again to get the third tier.
I believe the chance for unjust and untrustworthy are 6%, so basically you have to roll that 6% twice to get one of the traits. So, after two rolls of the same trait, you actually get the trait. The way multi-tiered traits work is that you have a % chance to get a point towards that trait, and it takes 2 points to give you the trait. You can get unjust and untrustworthy (each having three stages) from executing prisoners.
Here's a little run down on some of the bad ones.
