In the first place, in these days when one can't avoid seeing "Low-T" and "Blue-Pill" ads everywhere one looks, it's a trivial matter to achieve an erection, and of course castration makes it much more difficult to identify perpetrators, since they leave much less genetic material behind.

Second, it's undoubtedly barred by the constitutional restriction upon "cruel and unusual" punishments in the USA, at least, and most places in the world less benighted than a few that come instantly to mind. In most of those, the female victim would be executed first, whilst the perpetrator might well get off without punishment, since he can allege her lack of chastity without fear of contradiction.

Moreover, one can easily see by wandering over to the FM site that castration is in itself a sexual fetish (or 'paraphilia,' to use the modern and less judgemental term), and there would probably be many in the world who would scheme about arranging things "just right," flirting with the arousing possibility of being caught and having their fantasy fulfilled by victimising women (and possibly children) on their merry road to being "punished."

In general, criminals who prey upon people, rather than mere sources of money, lack both empathy and imagination, and most have an uncertain relationship to the mathematics of probability. People who break into vending machines and saw the tops off of parking meters are a different class of criminal than those who terrorise women for their purses or break down front doors and point guns at people to extort money and jewellery.

Indeed, after looking at the number of volunteers for execution, and waivers of the right to appeal a death sentence, one might hazard the guess that at least some "murders" are an elaborated scheme of "suicide by cop." It's entirely possible that the death penalty for these individuals is an inducement, and not a "penalty" at all.

One must always be aware of the "Law of Unintended Consequences." People are complicated. Some of them are not very nice at all.

Many modern "rampage killers" appear to have a similar suicidal motivation, but far more spectacular and deadly. Oddly enough, our modern concept of "rampage killing" goes back to the days when we Westerners encountered it on the Maylasian peninsula, where we encountered the culturally-defined practice of "running amok." This was actually still available as a diagnosis in DSM-IV, although I don't know if it made the transition to DSM-V.