Well, first my view about "view as guilty-> shoot immediatly" vs "10 years in death row, then kill" (like in the USA afaik):
In the first case it's very cheap, but you get a LOT of innocents, while the latter protects innocents better, but is quite expensive. There's also a sliding scale in between. Always keep that in mind while arguing about the price.
Well, my view on the matter itself:
pro death sentence:
feeling of revenge makes it less painfull for the victims/relatives of the victims
dead people can't endanger anyone
contra:
if the government shows that killing people is ok, even good, the society will learn this as well. As a result, people will be more ready to kill themselves on average.
it's unavoidable that innocents are killed (though it's unavoidable that innocents are imprisoned.. it's a matter of severity)
more expensive (if you try to weaken this argument, you are probably strengthening the one above)
if people know death sentence awaits them, they got nothing to lose.
neutral/refutable:
prevention - people rarely commit crimes in the believe to get caught, especially in the case of severe crimes ->little to no effect
cheaper - not true, or only true in case of endangering innocents (strengthening the counterargument above again)
"the executioner has killed a person, and needs now to be executed as well" - murder =! killing people
"killing is always wrong" - i don't believe in this, it's justifiable in self-defense, and certainly elsewhere
it's justice (eye for an eye..) - don't believe in this either
All in all, while being neutral if you exclude the first contra argument, this argument is sever enough for me to be against any death penalty, in any case.
I had discussions to solve this particular problem, but we didn't get a workable solution.
So, I'll probably go back to lurking.