Well no. I don't think that this should be the reason. It makes altogether too much sense and removes all morality from the matter. It also lacks any zeal or conviction. It will also unfortunately convince nobody.
The real reason is that you should respect life. You should give it your utmost respect. Taking away a life when it's unnecessary, speaks of a lack of respect. Innocent people getting punished unfairly is a terrible thing, but it happens all the time. Justice systems are imperfect. And removing the death penalty won't change that. Maybe it will make injustice slightly less permanent but that's about it. It will however make a symbolic point. That you can't morally justify murder. That's probably a better deterrent to murder than harsh punishment. If 'eye for an eye' is the standard for the state why would you expect the people to do any better? Why would you not expect them to say, carry arms all the time and get provoked into making a headline when they have a bad day? It's all connected. Leadership sometimes means leading by example and a firm 'no' to murder implies a 'no' to the death penalty.
The real reason is that you should respect life. You should give it your utmost respect. Taking away a life when it's unnecessary, speaks of a lack of respect. Innocent people getting punished unfairly is a terrible thing, but it happens all the time. Justice systems are imperfect. And removing the death penalty won't change that. Maybe it will make injustice slightly less permanent but that's about it. It will however make a symbolic point. That you can't morally justify murder. That's probably a better deterrent to murder than harsh punishment. If 'eye for an eye' is the standard for the state why would you expect the people to do any better? Why would you not expect them to say, carry arms all the time and get provoked into making a headline when they have a bad day? It's all connected. Leadership sometimes means leading by example and a firm 'no' to murder implies a 'no' to the death penalty.