In every list of loveable babyfaces in wrestling, the name of John Cena can always be seen. He remained a hero of children and grownups for over 21 years, he had also been the face of WWE for a long time. His name will definitely be written among the greatest wrestling in the history books.
But there had always been a constant request for a Cena heel turn and it started to rise mainly from 2009 as a large number of fans were bored with the same character for six years. But in 2008, WWE started a PG product and they needed a hero Cena, and he never turned heel for more than 21 years… until Elimination Chamber last Saturday.

John Cena’s Heel Turn is Compared To These Historic Heel Turns From The Past By WWE Hall of Famer
At the mentioned event, Cena triumphed in the men’s Chamber match and earned the right to face Cody Rhodes at WrestleMania for the Undisputed WWE Championship. But at the end of the event, Cena became a villain by attacking Rhodes after getting a signal from The Final Boss The Rock.
John Cena turned babyface back at Survivor Series 2003
WWE Hall of Famer Bubba Ray Dudley compared this iconic heel turn with shocking heel turns from WWE history like Andre The Giant turning heel before WrestleMania 3 in 1987 and Hulk Hogan turning heel at the WCW Bash At The Beach event of 1996. Here is what the Tag Team Specialist had to say;

John Cena remained a babyface for more than 21 years
"You can't waste this stuff on a tweener or somebody that's not [liked] ... people love Cody Rhodes, and people want to hate The Rock and The Rock's doing the best job he can do becoming hated. Once Rock came back and stuck his nose in Cody's business a couple of weeks ago, again, people started to chant louder for Cody.
“It's turning a loved ... Andre was loved all over the world? 'How are you going to turn Andre heel?' They did. John Cena loved all over the world. 'How are you going to turn Cena heel?' They did because at the end of the day it's easy; it's easy to turn John Cena heel, it's easy to turn Hulk Hogan heel, and it's easy to turn Andre heel because they're loved so much." Concluded the former two times TNA World Champion.
H/T - Wrestling Inc.