Ok, so I've thought about this some more, and I think I may be wrong. But I also may be right!
Let's put it in terms of Intent and Command. When you're using Investiture (at least, in certain ways), both of them matter. I believe the same applies to morality. For example, if you kill somebody who was intending to kill you (the Command), it makes a big difference whether you knew that they were trying to kill you, or if they were doing it for unrelated reasons (the Intent).
This was obviously premediated on Jasnah's front. But I think it makes a difference what she was thinking when she was premediating it. Would she have done this even if she wasn't trying to teach Shallan a lesson? If she would have, then I think it's morally acceptable. Her Command - her murder of the men - almost certainly ended up saving lives (at least, with my limited knowledge of the situation in Kharbranth), and her Intent was, indeed, to save those lives. But if she was just trying to teach Shallan a lesson, her Intent wasn't to save lives, it was to make a confused girl think about things. Which definitely isn't worth the lives of the three men she killed.
Fortunately, with Jasnah flashbacks in the back half, we might get to see her Intent.
Unfortunately, we won't get to see that for at least 17 years.
*sigh*