Death Metal is actually named after the band "Death", and is in fact nothing to do with imagery or lyrical content...
"Chuck Schuldiner" was the singer/writer (he's basically the band) for "Death" and is commonly referred to as the "Godfather" of Death Metal.
Previous to Death Metal, there was "Doom" metal, which is responsible for most of the imagery and lyrics in Death Metal. First "Doom" metal band? Venom. Though they obviously link directly to Black Sabbath...
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There you go, Death Metal 101. Because no-one cares