Btw to clarify my position: I fully believe that the world would be better off without certain individuals, and I do believe there are things that are inexcusable and worthy of capital punishment (or life on devils island like in the old days).

It's just that as a systematic approach it is flawed because humans will either make mistakes, or pervert the system. There are examples enough of wrongful convictions that we are not talking hypothetically.
And that price is too high imo.