my theory about the ending *contains spoilers*
So, I think this movie speaks to the idea that circumstances now are not so different from times of slavery and Jim Crow.
When she sees the drawings and gets angry and agitated, she is realizing that she has figuratively been moved off the plantation field and into the master's house--but she still has second class slave status. Slave owners often had a view they would "take good care of" their slaves.
Ok obviously she is not a slave. But there is an analogy there that resonates. He has been a weapon in the machine of oppression (he also has suffered, and is a sort of slave in his own context). He's been part of the machine that imprisons and kills black folks. Now she's taken into his house. Will it be a life of dignity? Will the bitterness of the fact he executed prisoners, and someone close to her ever go away? Can she really live with that, sleep with that at night? Or will she feel like an Uncle Tom?
I think she is volatile at the end. But then she sits down with him and has some ice cream. There are the gravestones. This is his last pitiful meal in his very own Monster's Ball. Last supper. I think she is planning revenge and to kill him at the end. Or at least, she is sitting with that power. Because of what he symbolizes and he is the manifestation of what has oppressed and destroyed her life until now. I think his gravestone is coming up next.
Not sure if this theory has been out there in reviews and whatnot? I thought it made a lot of sense.