There's this fancy Western European idea of abolishing death penalty. Wikipedia even has a nice map, with blue countries having abolished death penalty entirely, red still carrying it out, and green/orange having it on the books, but not in practice.
But reality is different than that. Most of the red countries almost never carry out the death penalty, reserving it for extremely unusual cases - in 2008 estimated 5727 people were executed, over 80% of them in mainland China, other than that only Iran and Saudi Arabia doing three-digit numbers. Other big red countries like India, USA, Indonesia, Pakistan, Japan and so on very rarely execute people.
As 59 million people die every year, your chance of getting executed is like 1:10,000, or more like 1:200,000 if you don't like in relatively execution-happy China, Iran, or Saudi Arabia.
Numbers like this are ridiculously uncommon - about 100 people die every year from peanut allergy in just United States. And "accidental suffocation and strangulation in bed", which sounds like a codeword for "BDSM gone awry" causes 327 deaths annually in just USA.
It's not just due to lack of crime - annually, half million homicides take place, and many countries have or had death penalty for lesser things. So if you're a killer, the chance of getting a lethal injection for it are 80:1 (assuming they get the right person), or more like 1500:1 unless you live in one of the three aforementioned countries.
Add to that the usual overconfidence criminals have, and it's clear that fear of death penalty is the least of criminals' concerns. Especially since even in United States you're over three times as likely to die as a result of a shootout with the police than due to legal execution, so we're talking at most 30% extra deaths factor here. The idea that people will stop carrying out crime due to fear of capital punishment is just highly dubious.
History lessonSo is this it? Did the abolitionists win? Not necessarily. Googling indicates that capital punishment for crime was never particularly widespread. Yes, governments killed loads of people - massacres during wars and occupations, killing political opponents, summary executions to quell social unrest, and some such. But legal capital punishment of citizen committing ordinary crimes after proper judicial process? Very unlikely.
Here's some statistics. Stalin killed about 60 million people, and Hitler about 20 million (estimates vary considerably, but let's take those numbers). How many actually received death penalties? Only 40 thousand from Nazis (1 in 500), and 750 thousands from Soviets (1 in 80). And these were mostly political opponents, war and civil war cases and such - not proper criminals.
How about other countries? Czarist Russia averaged less than 60 a year, and remember that many of them were political cases. Between 1770 and 1830 England had allegedly bloodiest criminal code on Earth, with death punishment for "writing a threatening letter", hanging out with gypsies, or "strong evidence of malice" and over 200 other crimes, while still averaging barely above 110 executions annually. Even the infamous Spanish Inquisition barely managed 20 burnings a year (but then, these were mostly religious and political opponents not criminals, so maybe they shouldn't be included here).
It's just really difficult to find any government, that routinely carried out large number of legal executions of criminals during time of peace.
When governments killThat's not to say governments don't kill. But these killings rarely affect common criminals. Nazis killed people for being of wrong ethnic group. Soviets killed people for being of wrong social class, or just because. The same 18th century United Kingdom which was so cautious with criminals of right ethnicity, killed million people for being poor and Irish (and no, it wasn't any Malthusian bullshit, Ireland was a major food exporter even during this politically caused famine).
Even countries which don't execute criminals at all like Denmark with no executions since 1892, change their mind during and briefly after war time, when 54 people were executed (it's not really a legal capital punishment, as it's all ex post facto). Likewise in Finland, after not executing anyone since 1825, about 550 people were killed during war time (unlike Denmark where it was mostly proper war criminals and people supporting Nazis, here you could get a firing squad for conscientious objection, and without even a real trial).
What does it mean for capital punishment debate?Other than perhaps in mainland China, Iran, and Saudi Arabia, the debate is purely speculative now. There is virtually no capital punishment anywhere. And other than during wars, occupations, major civil unrests and alikes, it was extremely rare for many centuries. Before that, executions might or might not have been more frequent, but it's hard to talk about proper legal proceedings, so they don't count.
People who support death penalty have a big problem, as there won't be any statistics showing serious effects of widespread application of it during time of peace, as it never happened. There are some bullshit statistics, like those in the original Freakonomics book (very entertaining book, by the way), but due to vast non-random annual variations in number of homicides and executions, we're talking about effects of a rounding error, and social science just cannot do anything useful with them.
So it's mostly about what makes you feel better. If you're a real Christian, then you should follow Jesus and be against death penalty. If you're a fake Christian like Southern Baptist, you should support death penalty because you hate black people more than you love Jesus. And if you're an atheist, do whatever you want, it's not like I can convince you.
Here, I solved it for you.