[quote=@Zoey Boey]
[@Lmpkio] I think Doom Guy is different from the Doom Slayer from Doom 2016. Doom Guy is just that pissed off marine on mars who goes around killing demons like a cool guy. Doom Slayer is some sort of ancient super warrior who is magical and Supreme Badass eternal murder man. 
[/quote]

Actually, according to Quake Champions of all places (which has the Doom Slayer armor), it suggests that Doom Slayer is indeed the original Doom Guy. It's a bit controversial atm, but it appears to hint such a thing, if not outright confirm it.

It's a bit of a controversial topic, but nonetheless that's where the signs appear to be pointing at.