Presumably, all the other people he kills will also be sent to Hell, given what they've done. That being the case, the punishment he puts them through before he kills them feels sort of redundant, since they will be subjected to much worse things for all eternity once they arrive in Hell. It's like, all punishments end up more or less on the same extreme level eventually. You might even argue that he's relieving their suffering somewhat by postponing their arrival in Hell when he puts them through some other punishment before death.