Surely you guys know why i'm asking this...
I do know that spiking a guy on his head is illegal and not only that it is also a very dangerous thing to do.
He could have broken Melvin's neck.
When I saw it I hoped he had not broke Melvin's neck, it looked dangerous as hell. As Toxic pointed out though, Melvin was in control of Varner he could have let go at any time to protect himself.
I think those throws Rustam Khabilov was using in his last fight were more dangerous, because he was trapping the guy's arm when he suplexed them. There are a lot of moves these guys do that could seriously injure someone.