1) There are endless cases of it being implemented on innocent people:
https://deathpenaltyinfo.org/stories/technical-errors-can-ki... or https://en.wikipedia.org/wiki/Wrongful_execution
2) Ethical reasons believing its wrong to kill people
3) Its well documented the process of death penalty has regular mistakes causing slow and incredibly painful deaths.
E.g. https://deathpenaltyinfo.org/executions/botched-executions