I don't think it should be about whether or not they are guilty though (although obviously, it's hard to bring the innocent back to life). It is about whether there is any deterrent impact (there is abolitely no evidence that there is) or whether it is about saying that state sanctioned murder is acceptable provided someone committed a "non-sanctioned murder". After all, not only are executions a form of murder, but there are other kinds of state sanctioned murders (have a look at how many times the US tried to kill Fidel Castro!) which apparently are "ok".
Then there is the disproportionate use of execution against certain
demographics - you are more likley to be on death row if you are not white or if you have a mental illness, and those demographics make up more than half of the executions in the USA, yet they make up much smaller percentages of the general population. Oddly, no white police officers who have murdered black people have ever been scheduled for execution - in fact it's damned unlucky if any of them are even charged with murder!
Execution is not justice, it is not humane, and it isn't even "even-handed". It is revenge, pure and simple, and reflects all the other inequalities in a society. Remember, this is a country willing to charge a doctor with murder if they perform an abortion.... Apparently though only "some lives matter" - and none of them are people of colour or those most vulnerable in society.