Italian gangsters are not fictional honor-bound archetypes found in the Godfather movies - they are vicious sociopaths recounted in Nicholas Pileggi’s Wiseguy, on the life and times of Henry Hill. OJ was no dummy - he would have known that they have have squeezed him dry if he hired them. That’s apart from the absurdity of being at the scene of the crime. The whole point of hiring someone is to lay off blame.
Lastly, professional killers would not practically cut her head off with a knife. She would have died from two gunshots to the head. Same with Goldman. From a gun with a suppressor.