It's called diplomacy. The North wanted to maintain the Union. Buchanan, himself, empathized a lot with the South on many issues. Anti-slavery Northerners called him "doughface" which was a nickname given to northerners who favored Southern political positions:
https://en.wikipedia.org/wiki/Doughface
It's not all as black and white as you're attempting to make it out.
Here is the full context of that quote: "
One heavy disappointment befell the colonel during the northward expedition. "I just missed the arrest of the notorious Osawatomie outlaw, Brown," he reported October 7th. "The night before, having ascertained that after dark he had stopped for the night at a house six miles from the camp, I sent a party, who found at twelve o'clock he had gone." Colonel Cooke was more successful in catching the latest overland immigrants":
https://www.kancoll.org/books/spring/s_chap09.htm
Cooke isn't say that he missed seeing the arrest of Brown, but that he had sent troops to arrest Brown at a house they believed he was staying at and by the time they got there, Brown had left. So, he was unsuccessful at catching Brown.
In post #691, you had claimed that he was meeting with "leading politicians", and that the government was keeping him safe. You named one politician he met with, and I named multiple who openly condemned him. Not to mention that it was the US congress that identified him as the leading perpetuator of the Pottawatomie massacre, if they were trying to keep him "safe", they would not have done that. The political North was varied, they did not all have the same views. James Buchanan was a Northerner who would ultimately support the Union, he still did not represent the views of all Northerners as he himself was not an abolitionist, and was very critical of the movement. Again, not as black and white as you are presenting.
Not all Republicans were/are politicians. And even if some politicians had been members of the Kansas State Committee of Massachusetts, that doesn't make the organization itself representative of the government. And the member Brown had been in contact with was not a politician.
"
Border ruffians were proslavery raiders who crossed into the Kansas Territory from Missouri during the mid-19th century to help ensure the territory entered the United States as a slave state. Their activities formed a major part of a series of violent civil confrontations known as "Bleeding Kansas", which peaked from 1854 to 1858. Crimes committed by border ruffians included electoral fraud, intimidation, assault, property damage and murder; many border ruffians took pride in their reputation as criminals." https://en.wikipedia.org/wiki/Border_ruffian
So, no, not every pro-slavery Southerner was considered a border-ruffian. And John Brown would have been known as a "Jayhawk."
Again, that was not proof. I already explained why it was not proof.