Just to give this information first, I am German and live in Germany and you probably know that we don't have the death penalty.
It's a very complicated issue and whilst there definitely are crimes extremely evil and mean, how will you make sure, that it's really, really, really, the right person who is killed for that? Would you really want to have the slightest possibility left that the wrong person might be killed?
I am very much interested in criminal cases and forensic psychology, there have been many cases in history in which the wrong person was convicted of a crime they just had not committed.
Death Penalty also does not adhere to the Ten Commandments. Thou shalt not kill - and I do think that if you do so, you're committing the same crime as the delinquent.
However, I can totally understand your feelings. In Germany, things are way different than in the US.
There's currently a guy in German media who is a pedophile piece of crap, who has a huge crime record, including having been imprisoned for some time and they let him go 'on probation'. Due to being known to the authorites, he was under surveillance for quite some time, and he had some clear restrictions (he was actually no longer allowed to get close to schools, he was not even allowed to speak with anybody younger than 18 years and so on). Whilst having to show up at the probationer's office regularly, whilst being under surveillance some time, he kept committing that ugly pedophile stuff.
He abused a six year old girl - massively - and that's only the tip of the iceberg.
Just by last month, he was sentenced to seven years in prison. This is, for Germany, an extremely long prison sentence compared to other cases of pedophile crimes. Sad, but true. Since he committed so nasty crimes and was so absolutely ignorant to any kind of (slack German way of) punishment and restrictions, he was sentenced to preventive custody as well - and is currently doing an appeal to get around that preventive custody.
He's guilty, he confessed, there are tons of evidences. I cannot imagine how the victims feel. (What German judges are doing at the moment is a slap in the face of many victims, there are way more examples than I can count- it's a shame).
If I would have anything to say, I would lock him up for the rest of his life. No light, no social interaction, no nothing. Water and bread once a day. No shower, no comfort. Nothing at all. He would go insane within 48 hours. And probaby kill himself - problem solved. For his crimes, he will burn in hell forever. At least, I hope so.