Just to note: Some death-metal bands have positive lyrics[0,1]... (Though, whether they can really be called a death metal band is possibly a point of contention.)
Avenged Sevenfold - Fermi Paradox: https://www.youtube.com/watch?v=vl8rofNa0tI
Avenged Sevenfold - Exist: https://www.youtube.com/watch?v=ntgOlFbGWLY
Nightwish - The Greatest Show on Earth: https://www.youtube.com/watch?v=IJb3xwtpmmc
Nightwish - Endless Forms Most Beautiful: https://www.youtube.com/watch?v=VUb1p8fm7Ag
Dream Theater - The Great Debate: https://www.youtube.com/watch?v=4z6vpiXQJNA
Allegaeon - Gray Matter Mechanics: https://www.youtube.com/watch?v=tu-NvEec8y8
Allegaeon - All Hail Science: https://www.youtube.com/watch?v=ONPMWHP8AZ4