A Comm for Historymemes

Surely you can see that attacking a military target with the intent to arm and free slaves is a completely different matter than slaughtering people for being merely associated with criminals.

More than just associated with criminals - supporting and enabling a coup with the intent of establishing a system of horrific human exploitation against the will of the majority of the territory.

The victims of the Pottawatomie Massacre lived on homesteads in Kansas.

As I pointed out, the legislature was fraudulent; you asked why it was fraudulent with this:

How so? To my understanding, Jayhawkers and Bushwhackers both migrated to Kansas in droves for the express purpose of voting on the issue of slavery. What was the fraudulent part?

To which I offered the reasons why the state government was illegitimate, not accusing the five killed specifically of not being residents; rather, pointing out that the state government that one of them was a part of and the other four vigorously defended was not only morally repugnant, but fraudulent as well.

Again, is guilt-by-association enough to condemn someone to a bloody extrajudicial killing?

If the judiciary itself in a country is subverted, what moral condemnation is extrajudicial punishment? Extrajudicial punishment is reviled because it operates on the presumption of a functioning judiciary; the judiciary in Kansas at the time was quite explicitly controlled by the pro-slavery faction, with little regard for actual democratic or legal processes, as John Brown had found out prior to the massacre.

Tell me this - would attacking the rear-echelon troops of a military be condemning men simply for guilt-by-association?

The cooks, the truck drivers - what offense did they commit, but to wear the uniform of an organization deemed to be offensive to life or liberty?

How much of this argument you're putting forth is principle, and how much is convention?

Convention has a reason - it is neatly organized moral judgement for complex issues that can be reliably and consistently applied - but it is reliant on prerequisite circumstances, circumstances which are usually, but not always present.

In Bleeding Kansas, the core aim of a major faction is to inflict indignity on a massive swathe of human beings for no crime other than their birth, the state apparatus is not being applied to prevent said faction from violating laws and norms in pursuit of this goal (and, in fact, is explicitly working to enable it), and the major supporters of the exercise of violence are not uniformed individuals or even organized bands, but in outbreaks of vigilante violence - of the kind John Brown et co visited on the supporters of said violence in turn.

Convention is not very useful in that context.