A/N to answer your question Railman :)

Flashback:

Curtis rarely made sense to Collins. His psychobabble washing over him like the shower he takes every few days. The Doctor was an intellectual inferior. But one thing stuck with him.

Love for Geniuses only comes once in a lifetime.

This certainly made sense to Mark. He had no desire for anyone else- never had done and though his desire for Walter had originally stemmed from the cerebral, desire him he still did.

Walter was everything he could've wanted in a person. He made him feel special for being special, he challenged him in ways that nobody else could and over time Mark began to appreciate how brilliant not only Walter's mind was- but his body too.

Walter had previously shown (to Mark's knowledge) zero interest in anyone. For all Mark knew he couldve been a virgin. As time went on Mark allowed himself to hope, believe that one day Walter would realise that nobody would match him like he could and Walter would be be his.

So when the Cheerleader appeared in Walter's life (admittedly from a distance) Mark experienced a new emotion for the first time in his life- Jealousy.

He began working tirelessly to mess with the others in Walter's 'Cyclone' to drive them away. He wanted Walter to forget they existed. Then he'd get rid of the Cheerleader.

But as he pushed them more the more they fought back, and by the time he decided to focus himself entirely on Walter he realised he was too late. Walter didn't know it but he did.

Walter had fallen for the Cheerleader.

He gushed over her beauty, her talent (as if wrapping her leg round the back of her head like a whore could be called a talent) and her EQ.

Walter had seen the ways she communicated with others in the way they couldn't and when Walter called her "a Genius in her own way" Mark seethed inside. She'd clearly sullied his mind.

Mark fell into dispare. Walter was lost to him and in time Mark lost himself. His mind whirled with brilliant but terrible thoughts to make Walter his. A awful yet genius idea nested in his mind and over time took over his entire being.

A lesser person would call it evil. Inhuman even. But Humanity was overated anyway.