Demetri didn't feel good in Sam's car. Not just because of how he couldn't defend himself from Eli and his new friends. Sam and Robby only knew half of what was bothering him.
Demetri had lost his best and oldest friend. Demetri cannot see how that happened. Demetri posted that yelp review because he believed it was the right thing to do.
Maybe Demetri shouldn't have been surprised. Maybe he should have seen this coming. He felt less close to Eli ever since he started taking those stupid karate lessons.
If this was a comic book Demetri would say Eli had been possessed by Hawk.
But this was not a comic. It was real life. And the truth was Eli was gone.
It was all Cobra Kai's fault.