My theory of the killing of the guy from the US
I have seen two conflicting reports of where the apartment is located. One shows a photo of the building where primarily gringos stay. The other (video report) shows an apartment across the street. I have stayed in apartments on that street many times. I have never seen a gringo going in or out of the apartment in the video. I think it is the apartment building in the photo. I don't think it is a first floor apartment because the report said the two tried to escape on the roof.
As to what happened, I don't think the age of the girl had any bearing. The report said the gringo was found naked. I think he and the girl had sex, and he was taking a shower afterwards. While he was in the shower, the girl went downstairs and let the guy in. They intended to rob the gringo, but he resisted. There was a struggle and the gringo realized he was in trouble and began screaming. That's when a neighbor called the police. They slashed the gringo's throat and tried to find money, which was probably locked in the room safe. They stayed in the apartment too long and heard the police sirens, so they tried to escape through the roof.
IMO, the fatal mistake was resisting a robbery.