Why did John brown attack people at pottawatamie creek in Kansas?

At that time John Brown was a Captain of a company of Militia of Kansas State, which belonged to those organized by the "freesoilers" in opposition to those organised by pro-southerners settlers.

When the village of Lawrence was put on fire, Brown was extremely indignant because he judged the reaction of freesoiler local government as too weak.

So he decided to react on his own initiative.

As he was reported that some freesoiler settlers, had been menaced and insulted by southerner pioneers, settled near the Pottawatomie Creek, Brown and his band sprang upon their house during the night, dragging and then killing five of them out of the house.