The 1864 movement is the one that caused much of the animosity. As I mentioned to Ken, the records of the various Kansas regiments indicate that there were constant raids out of Fort Smith throughout the last two years of the war. These inflicted much misery on the Choctaw people and generated considerable resentment among them.
There are a couple of things about Poison Spring that should be kept in mind. First, a review of service records and muster rolls indicates that casualties there were quite a bit lower than traditionally stated. Most of the missing in action actually returned to their regiments within a month or two of the battle and the number of killed and seriously wounded was quite a bit lower than is stated in some accounts.
Second, Union officers explicitly say that the men of the 1st Kansas Colored Volunteers, which suffered 117 killed in the battle, refused to lay down their arms but insisted on carrying them away from the battlefield. This led to severe casualties among them and at least raises the point for discussion that they fought to the death rather than surrender.
Third, the Union commander at the battle states in his report that many of the men of the 1st Kansas found themselves without officers at a key stage of the battle, a fact that resulted in great confusion in their ranks at a point when they were under heavy fire in both front and flank.
Fourth, Union officers at the battle did indicate there were reports of Confederates shooting wounded men, but at the time they made no claims of a massacre. Instead, they reported (incorrectly) that they had inflicted twice as many casualties on the Confederates as they had received. The massacre claims grew after the battle, but were not a point of discussion immediately after the fact.