In Antebellum times, if a poor white person murdered William Ellison in 1860, I think that the poor white person would likely not be charged with a crime unless the circumstances were such that the killing of Ellison was "a recreational killing." If the circumstances were any type of feud, I think the poor white person would not be prosecuted.
I am not sure why you think that the standard for a master to be convicted of killing his slave was the same as for killing a free black.
I am not sure that free blacks' status was that much lower than whites in ante bellum times. There wasn't so much of a Jim Crow system then. There wasn't as much of an issue about race then, when it was clear who was in control. There weren't many lynching then. There also wasn't as much of an issue about involvement of black men with white women as later on.
Most blacks became free by their fathers freeing them or by buying there own freedom or having their freedom bought by a free relative. So likely many were wealthier than the average white person.
You also say the deep south. As I indicated, most of the free blacks in the south were in MD or VA and most of the rest were in NC, DE, or LA. So by far the most cases of that happening would be in one of those relatively moderate states.