1) Executing someone ends up costing more than incarcerating them for life due to all the appeals.

True. That’s why there should be as swift a resolution as is possible, done with the “absolute confidence” not the “reasonable doubt” standard. Plus two mandatory appeals to judicial panels, not repeated and prolonged jury trials. I also suggest taking plea bargaining away for anyone involved in a mass murder (i.e. more than one person).

2) The death penalty has never been shown to be an effective deterrent.

Sure it has. It stops murderers from killing someone again, either in prison or after escaping or being paroled. All of those things have happened repeatedly; see my link upthread.

3) Innocent people get executed.

Innocent people get murdered first. And while what you say is undeniably true, we get people killed all the time in the name of law enforcement, or war, or forcing them to do dangerous things to survive, or even reality television. That last one is a joke, although it is also true.