According to an interview in The Daily Mail, it was HHH himself.
When asked if turning Cena heel was at the top of his to-do list Paul Levesque said:
"For me? For sure. You always want to approach things like the 'what ifs'. You have a blank slate and John's amazing."
When HHH was asked if it was hard to convince Cena to turn heel Levesque said:
"No. It was a conversation - without getting into the details - it was a conversation that when it was brought up, everybody... I think in John's mind he was like 'Wow, I didn't think you were going to say that. I love it.' It's a challenge to him. John's not a complacent person, right?
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