Ha - that's funny. Or like ironic at least. How do you figure that Araya is "pretending" any more than Varg (doesn't that mean 'wolf' in some language? What a fake)?
I wager that Araya has a deeper understanding of his own background and religion than wolfman does. Exploration of darkness is the essence of Catholicism - especially for American Hispanic Catholics. Perhaps Varg neeRAB to experience life outside of the liberal 80s Scandinavian politics that spawned the region's fairly ridiculous 90s black metal scene and a posh jail cell before he can ponder what the meaning of 'extreme' is, eh?
If you're just asking why Slayer is a good metal band then here is my response:
Araya screamed with more hardcore punk finesse than any of the other 'big ones', Lorabardo played better drums than pretty much everybody since the Jazz greats, and King played ingeniously unintuitive, nonsensical guitar solos that stir the soul more than any xenophobic Norwegian asshole ever has. If you like Wolfie but hate Araya, and this bias reflects in your metal taste, then I doubt you're much of a music fan.