The article also misses another point: when blacks kill blacks, it's frequently also a criminal killing a criminal. Juries aren't in a big hurry to execute someone who's killed a drug dealer or a gang thug.
You've made a good point. There are a number of factors that determine whether or not a murderer receives the death penalty: criminal record, the circumstance and level of brutality involved in the killing(s), whether or not the victim was an upstanding citizen, the killer's remorse or lack of it, the killer's motive, state of mind, mental competency, etc.
I can't even begin to explain the disproportionate number of black criminals. I don't think it has anything to do with race, though. I don't think that the blacks who commit crimes do so because they are black. I think it may be because they've embraced a particular subculture.
There are a number of subcultures in America, one of them being the inner city, product of a single parent home, gangster subculture. I'm not saying that people aren't responsible for their actions and choices, nor that people can't overcome environmental influences. There are a number of reformed gangbangers and a number who never go down that path in the first place in spite of the overwhelming pressures to do so.
I've read that prior to LBJ's "War on Poverty" that blacks were well on their way to becoming middle class and upper middle class citizens. That the destruction of the black family, the diminishment of the incentive to work hard and strive to get a good education became rampant after LBJ's socialist meddling.
As a teenager, I had a friend whose mother was on welfare (she was white, if that matters). Because her mother was on welfare, my friend was not allowed to work. If she got a job flipping burgers like all her friends, her mother would lose welfare benefits. She really wanted to work, but her only choices were to remain idle, work illegally for cash "under the table" (or engage in worse illegal dealings to obtain money), or throw herself into her schoolwork, after-school activities, volunteer work, etc. She was a good kid and chose the latter.
Many choose idleness or crime and end up on welfare like their parent or they end up dead, in prison, or criminals at large. It's a vicious cycle. Nobody wins, except the politicians, who "compassionately" wrap the poor in chains.