Wood chipper is to humane. To quick a death.
They need to be put on a 500 yard field, up hill slope after getting their junk cut off by the fathers of the victims using whatever implement they choose. Scalpel to a dull pocket knife, hack saw, 21 ounce hammer, propane torch, cheese grater, the sky is the limit. I can think of plenty of interesting methods for females as well, because they exist as well. Checked up pipe tap, wood spade bit, red hot iron rod, you get the idea. No drugs, no anesthesia. That 500 yard slope ends in a sheer drop to shattered rocks at the oceans edge. No place to hide, only option is to run away from the family of the victims or victims themselves as they shoot at them. If they live, they can stand there naked with their chopped or mutilated junk and wait to get shot off the edge, or jump to their death after falling for thousands of feet. No recovery of the body, just a red splat to feed the birds and fish.
It's to cruel though in democratic eyes. In my opinion, it's not cruel enough. Nothing exists that is cruel enough. Maybe I'm a psychotic sick fuck. Maybe it's because their lives are meaningless and they don't deserve to live, and the devil just needs to be shown exactly the treatment these worthless wastes of oxygen deserve.