Last week, viewers saw Theresa McQueen (Jorgie Porter) unveiled as the killer of Whittle's character, bringing a six-month long whodunnit storyline to a close.
Theresa committed the crime after being left devastated when Calvin indicated that he did not want to support her or their unborn baby.
However, speaking to PA, Whittle confessed: "My opinion was it would have been someone else. If I was going to pick a killer for Calvin I don't think anyone had a strong enough motive apart from Mercedes, Malachy and Spencer.
"Obviously, with Malachy, I was sleeping with his wife. Mercedes is a woman scorned, and I don't think Spencer understands consequences so he could probably act without realising that if he shot Calvin he would die and that would be forever."
He added: "I thought it could possibly been him because he was upset that Calvin left Warren to die, so those are the only three I felt had enough motive to pull the trigger."