He began his inquiry by ingesting all the words at www.darklyrics.com, "the largest metal archive on the Web," according to its front page. After cleaning the returns and filtering the languages he had a dataset comprised of 222,623 songs from 7,364 bands.
Here's a visualization of the most frequently used words from that corpus, removing all of the most common english words (stopwords):
Little surprise there. "Life" and "time" recur most often. Metal is angsty and its brooding usually bears that high school-age existential flavour. In fact, all your Family Feud guesses are there: dead, blood, soul, night, hate, pain.
But to find the most metal words, he'd have to find which words appear more frequently in the word bank he collected than in everyday English usage. Here's where I'm going to have to trust his math.
For this, he turns to the Brown Corpus, a collection of 500 English language texts, totalling about a million words, sampled from a variety of sources in 1961 by Brown University linguists. It's widely-used in the field of computational linguistics because it's relatively representative of English usage.
He compares the frequency of word occurrences between the two texts (filtering out words that appear fewer than five times) using a logarithmic function. If you're the sick sort who liked it when teachers said "show your work," you can read more about it here.
And there you have it, these are the 20 most and least used words in metal:
Most Metal Words
Least Metal Words