Denial of human rights. Sentencing someone to death denies them the right to life – enshrined in the Universal Declaration of Human Rights.
Irreversible and mistakes happen. Execution is the ultimate, irrevocable punishment: the risk of executing an innocent person can never be eliminated. Since 1973, for example, 150 US prisoners sent to death row have later been exonerated. Others have been executed despite serious doubts about their guilt.
Does not deter crime. Countries who execute commonly cite the death penalty as a way to deter people from committing crime. This claim has been repeatedly discredited, and there is no evidence that the death penalty is any more effective in reducing crime than imprisonment.