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Wait wait wait. Was the Metal Masked Assassin supposed to be Nathan's foil?
They're both big guys with long hair (in opposite colors even). They both speak with a natural death growl. Both leaders of some sort.
The MMA is like what Nathan wants to be early on in the series. He's brutal, he never shows any emotions except rage, he doesn't care about a single person on Earth now that his brother's dead. Metal is seemingly fused to his face, he literally can't stop being "metal." And that's the way Nathan tries to describe himself and his band, especially in interviews. They have rules about not caring about people. They can't like things that aren't "brutal" enough. What he wants, when he's young and immature, is to be just like this man.
But Nathan's not like the Assassin, and that's what makes him a good guy.
Nathan cares about people. He loves his bandmates so much, he started thinking of them as his family. He tries to have a good relationship with his parents, even if he thinks they're embarrassing. He realized there can be value in being soft, and in reaching out to people.
Consider the only dialogue they exchange in the entire series:
MMA: You took my brother from me. NATHAN: But we'd rather burn in Hell than let you take ours.
The MMA didn't know what happened to his brother until General Crozier told him. It was a surprise, because the two didn't really seem to be that close, and he didn't know what his brother had been doing. But Nathan's always around Toki (and the rest of Dethklok). When his brother was taken, Nathan and the others found Toki using Magnus' clues, which they could only do because they pay attention to each other. They still remembered the place where Toki had his first show and his audition many years later.
And after that one exchange? THE MMA DIES. He's gone from the rest of the series, because he's already served his narrative purpose. The MMA's line shows that he's going to remain stagnant forever, single-mindedly pursuing brutality and violence. Nathan's line shows how far he's come, that he can confidently say how much he cares about someone. Nathan overcame his inner conflict so hard that his foil STOPPED EXISTING.