Mike looking so much like William is SO important, actually, because it adds so much to the quiet horror of it all.
Young Mike taking pride in looking like his dad, because it's the only thing that William seems to love him for. Elizabeth is his favourite, the golden child. Evan is either ignored or coddled when he cries. Mike learns from a young age that his father only cares for him when he sees that he can get something out of it; if William looks at his son and sees a reflection of himself, maybe, just maybe, he'll start to love him more.
Then after the bite, Mike starts to hide his face. Because his father no longer loves him for being his mirror image. He resents him for it - because despite the fact the bite never would have happened if William hadn't pitted his children against one another, he blames Mike wholeheartedly for taking away something he perceived as his.
Mike looking like him is no longer endearing. He's no longer a potential successor. He's a rival. He's a threat, because William looks him in the eye and fears seeing his own evil looking back.
After William disappears, after he's accused of the murders, Mike stops wearing his mask. But he can't look in the mirror. Not anymore. He spends years carefully avoiding his reflection. Because he's afraid that if he does, he'll see his father grinning back at him, and the one thing he fears more than anything is turning out like him.
Being scooped is harrowing. It's the worst experience of his life. But in some ways, at a certain angle, in some quiet, hidden part of himself tucked away in a part of his mind Michael doesn't dare to unpack, he's grateful - because at least now, no one will ever mistake him for William again.
