Death is irreversible. Imprisonment is not. Unfortunately, US criminal system routinely imprisons innocents (and in other countries I'm sure it is not much different). E.g. check out Innocence Project[1]. Criminal justice system is rife with errors, malfunctions and abuse. For imprisonment and fines, one can at least fix an error after the fact. One does not get back years spent in prison, but at least they can get free and maybe also compensated for it somehow. But if the person is dead, you can not fix it.

I was for a long time of the opinion that there are crimes deserving death penalty, and I still kind of think so. But - I am no longer sure the US government, and most of other governments, can be trusted with this power. They are too broken and dysfunctional to be able to wield it justly. As such, it is better to not have it at all.

[1] https://en.wikipedia.org/wiki/Innocence_Project