Here is the full context of the letter. It is a request from Chief Harkins to the American Public for kindness and justice. He uses the letter as an opportunity to eloquently explain the conditions of removal, and provide historical context in order to counter myths and correct the record.

Believing that our all is at stake and knowing that you readily sympathize with the distressed of every country, I confidently throw myself upon your indulgence and ask you to listen patiently. I do not arrogate to myself the prerogative of deciding upon the expediency of the late treaty, yet I feel bound as a Choctaw, to give a distinct expression of my feelings on that interesting, (and to the Choctaws), all important subject.

In this context, the address seems genuine. He is not blaming all American citizens for the outcomes his people face, but specifically seems to be concerned with the legislators (not the citizens) of Mississippi. With that said, he also speaks to the legacy of harmful acts committed by settlers and governments against his people. The letter is a farewell letter, and does seek to appeal to the better natures of Americans - but it stops short from letting them off the hook for crimes committed.