It was in the news Sunday that so far this year there have been 11 Texas inmates murdered by other prisoners. During the same period of time, there have been 15 legal executions in Texas. This just goes to further show that capital punishment in the US is already "very rare, if not practically nonexistent" if the practical purpose is to deter crime; a criminal faces risks as daunting or more daunting than the prospect of being apprehended, convicted, sentenced to death, and exhausting his appeals.
It might also be pointed out that eliminating capital punishment does nothing to eliminate "extrajudicial killings" or "justified homicide" by police. If you think that in today's post-9/11 environment no death penalty would not be a word game meaning no trial before the killing, with quite probably more killing actually being done than in the old days of the death penalty, you're fooling yourself.