The real truth is that he was cursed by his own mother. Basically, He's only allow to kill once, after that she's gonna kill him IIRC. The reason why is because his mom is basically the Grim Reaper and having a son going on a killing spree is gonna cause a lot of issues with work. So he needed the shackles. And lets face it. If she didn't put that shackle in place she would have had to deal with a lot of his victims and she wasn't having any of that bullshit.