It took 8 minuted for him to die 3 minutes of which he was unresponsive while people warned the cop and asked him to stop. The victim informed the murderer of exactly how he was being murdered and asked him to stop. He called out for his mother then stopped speaking at all while he died in silence. The method he was being killed would have been completely obvious to anyone who possessed a pair of lungs or understood how breathing worked.
Nobody gives precisely one hot damn what manner of training he received. It was obvious he was murdering his victim to a human of ordinary capability. The logical conclusion is that he didn't care or wanted to murder him.