a) she made the decision to sell him to the mining company. she did this, further, knowing full well that
b) he was a free man.
so, i think there are at least two reasons why she is a villain beyond simple "complicity" arguments.
II. Necessity of her death
a) she would otherwise have been the only living witness to the sale to the mining company. absent his bill of freedom (which had to have been taken from him or destroyed to facilitate his sale to the miners) and given Dr. Schultz's death, there was no proof that he was a freedman. this would make the sale to the mining company legal.
b) the word of a wealthy, respected white landowner and regional figure, who just had her brother murdered and household executed by this same guy...who has no proof that he's not a slave? and has a runaway brand on his face? at the very least she could insist that he was not a freedman and return him to slavery.
so, i think there are at least two reasons why her death was necessary.