Well I want to start by saying that I don't want you to just drop your views, but I want to add my own and hopefully it will make some small difference.

Killing humans, in my own opinion, is wasteful. Humans are invaluable resources. For one, rapists can repeatedly donate parts of their liver (only for it to continually grow back) over the coarse of their lives. They can also submit to drug trials, psychiatric and biomedical experiments and other research arrangements that no layperson would ever consent to. Is it not better to turn this damaged person into a tool for the good of society? What if your own child goes crazy someday and rapes someone? Would you be willing to kill them?

Why exactly is it better to kill than to imprison? We are also killing the possibility that this person might actually be rehabilitated in the future through some as-yet to exist treatment. You may tell yourself that you have a right to act on your emotions, but then how is this any different than what the rapist did? It's a slippery slope. If we want to set an example, we shouldn't use vengeance or hate as an argument. Many rapists will argue that they feel disrespected and are avenging their own hurt egos by raping others. They will see eye to eye with you if you react to their crimes in the same manner.

One last argument: punishment does not work. It never has. If your goal is to bring this person to justice, you will be sorely let down. It doesn't make rapists feel bad because they don't think what they did was wrong in the first place. Punishment doesn't make victims feel better; in some cases media coverage of long-drawn out trials can haunt them. It doesn't make society any safer - it clearly doesn't deter crime.

If we kill people, for any reason other than the immediate protection of the lives of others (and even this is debated by many), we are sacrificing our own good characters and becoming exactly the kind of person we despise so much.