As Bill Clinton recognized, the anti-death penalty crusade is a loser for the Democrats. The small minority who passionately care about it are solid in the Dem camp anyway, so there's no need to cater to them.
And they were largely successful selling Trayvon to the public as a young sweet boy buying "IT and skillets" who was gun downed by a racist white Mexican (a new racial category, created specifically for this event.)
They're not going to champion this mangy dog murderer for exposing some technical issues with the humane death induction machine.
In 1992 Bill Clinton interrupted his campaign to go back to Arkansas so that a prisoner could be executed—and I think it was a black man.