Him

The goat-costumed attacker was Malek, Isaiah's trainer, orchestrating the attack as part of a predetermined plan to lead Cam to the Saviors. This assault was deliberately arranged so Cam would suffer a brain injury that would make him vulnerable to Isaiah's manipulation and recruitment into the G.O.A.T. program.

The G.O.A.T. lineage is a secret group of manufactured football players who gain enhanced athletic abilities through ritual blood transfusions passed down through generations. Each potential successor must undergo the blood transfer and fight the current G.O.A.T. to the death to determine who receives the power.

Isaiah trains Cam as part of the G.O.A.T. succession process. He needs to evaluate whether Cam is worthy of becoming the next G.O.A.T. by testing his physical and mental toughness through brutal drills, while simultaneously conditioning Cam to accept the blood transfusions that will eventually transfer the G.O.A.T. power.

Elsie seduces Cam as part of the psychological manipulation central to the G.O.A.T. program. Isaiah later uses Cam's guilt about cheating on his girlfriend against him, ridiculing him for feeling remorse and teaching him that winners must be remorseless, conditioning him to embrace the ruthless nature required to become the next G.O.A.T.

Marjorie was a deranged Isaiah fan who tried to intimidate Cam out of replacing her idol. When she attacks Cam in the sauna, Isaiah intervenes and kills her to demonstrate absolute loyalty to the G.O.A.T. program and to show Cam that anyone who threatens the succession process must be eliminated without hesitation.

The red liquid is Isaiah's blood, the same substance used in the G.O.A.T. ritual. The owners force Cam to drink it without his knowledge, initiating the blood transfer process that will eventually allow Cam to receive Isaiah's enhanced abilities and compete for the G.O.A.T. title.

Marco, the team doctor, knows the true nature of the G.O.A.T. program and its violent requirements. His warning to run is an attempt to save Cam from being drawn into the blood ritual and forced to fight Isaiah to the death. Marco is later killed and his severed head is shown to Cam as a warning.

After Cam unknowingly drinks Isaiah's blood at the party, he wakes connected to an IV of Isaiah's blood in an ice bath, this is part of the ritualistic preparation for the G.O.A.T. transfusion. The cold temperature and blood transfer are meant to begin the process of transferring Isaiah's enhanced abilities to Cam before their death match.

Cam's father arranged for his son to become the next G.O.A.T. player from the beginning. The entire sequence of events, Cam's rise, his injury, his recruitment by Isaiah, was orchestrated by his father, who sold Cam to the Saviors' organization as the next successor in the G.O.A.T. bloodline, continuing the cycle of sacrifice and power.

According to G.O.A.T. lineage rules, only one player can possess the enhanced blood powers at a time. Isaiah reveals that Cam and Isaiah must fight to the death, with the victor receiving the other's blood through transfusion, thus inheriting all the accumulated power and becoming the new G.O.A.T.

Tom is initially spared by Cam during his violent outburst because he was the one who told Cam about Isaiah's training offer, showing Cam mercy in gratitude. However, after the rampage, Tom is dragged into a pentagram and destroyed by an unseen force, suggesting the G.O.A.T. organization has supernatural protection that eliminates anyone who helps potential successors escape.

Cam survives by embracing the violent, remorseless nature required of the G.O.A.T., but the ending is bleak, he has become corrupted by the program, having killed everyone around him including his potential allies. Walking bloodied into the desert suggests he is now trapped in the G.O.A.T. cycle, either as the new successor or as a broken survivor with no way out.

The masked attacker revealed during Cam's violent outburst is Malek, Isaiah's personal trainer. Malek was the person in the goat costume who attacked Cam at the beginning, orchestrating the brain injury that set Cam on the path to the G.O.A.T. program. He was part of Isaiah's inner circle throughout.