But you may need testicles to be motivated to re-offend.
Only if you think that it's a sex crime. It's a power crime - the offender has power over his victim. You don't need testicles for that, or ovaries for that matter. You need a functioning brain, and a way to exert power. Castrate him, and he is still capable of thought, he still has hands, and he can still speak. He's still a danger. As long as he can think, he's capable of the motivation. It's not like neutering a cat so he doesn't spray, or having your dog fixed so he doesn't wander. Kill him, and you remove any chance of him ever doing it again.