There are many good reasons to eliminate the death penalty.
The primary one to me is that our justice system is seriously flawed. Police lie, prosecutors lie and withhold exculpatory evidence, judges are variable, juries can be swayed, eyewitnesses are notoriously inaccurate, forensics are often based on crap science and effective legal help is not available if you are poor or even middle class with limited resources.
The Innocence Project has effected hundreds of exonerations for both death row and life sentence inmates who were wrongly convicted. It estimates that at least 1% of inmates are wrongly convicted and that equates to over 20,000 people.
And most do not get overturned due primarily to limited resources, destroyed evidence or just bad representation. Without DNA evidence, which are the majority of crimes, the odds are terrible no matter how egregious the prosecutorial abuses are.
It is immoral to have an irreversible penalty with a flawed system of justice.