Like other forms of English, Death Metal English is a tool kit. It can be used well or poorly; Vastum use it quite effectively. You can employ Death Metal English in other realms, but it’s designed to make death metal lyrics sound more brutal. Why do DM bands write this way? Perhaps because a few early tastemakers chose to, and everyone else just followed along. Perhaps because all those lengthy words resonate nicely in the front of your face when you utter a death growl. Perhaps because its ponderous diction mirrors the blocky nature of the music. My best guess is that death metal bands use Death Metal English because it’s innately awesome.