While I am horrified by what happened, we have to look clearly, without our emotions clouding things.
This guy is a Jew. A sick one, but still a Jew. While a secular court is halachicly allowed to impose a death penalty, I don’t know how it applies when the criminal is a Jew. I believe halacha should take precedence.
Even if this man met all the requirements to be sentenced to death by a beis din (which our botei dinim now can’t do), remember that a beis din which carried out such a sentence more than once in 7 (or 70) years was considered bloodthirsty. Let’s not be bloodthirsty and call for his death.