There's numerous ethical reasons for the abolishment of capital punishment. The two the stand out the most to me are:

1. Wrongful convictions - People have been convicted and executed and later exonerated with new evidence. People are wrongfully convicted with way too much frequency. It is far better if the person is still alive to find some quality of life after the state takes years from them than for them to have their name cleared after the state has executed them.
2. Is the purpose of a given sentence deterrence, rehabilitation or state-sponsored revenge? I am of the opinion that criminal convictions should be focused entirely on rehabilitation. Take the person out of the situation and environment that caused them to commit the crime, focus on providing them education and services so that they can better help themselves avoid recidivism, and ultimately release them in a timely manner for a second try as a member of society but now with the tools and knowledge they lacked before to handle the bad situations that pulled them into the mess. Save locking people up for decades for those who re-offend and continue to commit heinous crimes without remorse

State-sponsored revenge will never provide justice, and it has been proven repeatedly that severe sentences do not deter severe crimes. Neither are paths to justice, only paths to more incarcerated people