Capital punishment can be a good thing, but not when applied by corrupt entities based on unjust standards and lies. History has shown beyond a reasonable, that when applied correctly, it can serve as a pretty good deterrent. In addition, not all killings are murders.
There is by deliberate design very little sympathy left for the victims of murderers today. The status quo is to feed and cuddle the murderer and start a fan club - it is all about his/her/its feelings and welfare. It is the sign of a sick and decaying society that won't survive if people don't come to their senses. Yet, many deem it as advancement to a higher level... until they and their loved ones are tortured and murdered in cold blood themselves. If someone cares that much for them - and I am in reference to murderers, not people falsely accused of being murderers - then they should spend some quality time with them to get a grip on reality.
Are all killings wrong though? Even when wars are started many people argue that it's still wrong. I'm not sure I agree with this opinion but it is something to consider. That also begs the question when is it acceptable to kill another. You could argue self defense but only a minute amount of cases actually require lethal force to make sure you are the one to survive.
The thing is people are getting sentenced to death when they haven't committed the crime and it comes out years after they are executed. This should be avoided at all costs but the only way of going about this is by actually abolishing it. If we look at some of the prisons around the world especially in Panama etc then the welfare of highly dangerous criminals isn't really something which is looked at closely and they live in horrendous conditions. Being falsely accused of a crime is bad enough to be sentenced to death is something else.
I think you outline most arguments in your OP and address them.
It is a difficult question society needs to ask itself.
I don't think that death penalty acts as an efficient deterrent. Spending 30 or 40 years in jail, or even a life sentence, seems like a very good motivation to not commit a crime. I don't think that simply by adding more punishment we get better efficiency and I think statistics back this up.
Losing a person in our society, even if it is a criminal, still impacts us negatively. We benefit from every mind that is alive today, if for nothing else, to tell their story.
There is one type of situation where I am thinking that death penalty might be applicable and that is if a person in question really is so dangerous and competent to hurt others even if sent to high security prison. These situations are so rare however, so I don't think that this makes any significant impact on the decision.
I would have to agree that a life sentence should be enough of a deterrent. Especially if you are in a maximum security prison. There's been studies on the effect of prison on innocent people who were falsely put in there and how it lead them down a dangerous physiological state so when people say doing time is easy then studies have shown otherwise.
I'm guessing you are referring to serial killers which are with out a down guilty. I'm not sure that a death sentence is still warranted though. Could you imagine living with yourself in a 4 wall cell with only your thoughts even if you were put into a cell for an experiment it would drive you to insanity.