my thoughts on the last 30 minutes.

First we see a leather clad guy topping a handcuffed guy, while his doberman barks and cries outside the door

Cut to the whole movie, about Sergio and his growing obsession with Joja. We end with Sergio handcuffed and crying on the ground (probably more about joja's rejection than being handcuffed)

Then we're back with the leather guy, which is revealed to be Sergio. Now his sex partner is on the bed not moving, not even when Sergio lets the doberman in. I think Sergio accidently killed that guy during some rough sex. Not like asphyxiation or anything, but maybe the guy was sick. The way the dog was crying, i think he may have been a medical alert dog, trained to react when they sense their master is going to be sick. The dog was nipping at his owner, but he was unresponsive. Even if you're dead tired from sex, you don't just lie there as your dog licks your naked backside and legs, you shoo him off.

I think Sergio loses it, he grabs the handcuffs and finds Joja's house again. As we saw, he ties him up, gags him, simulates sex, pushes him off his 2nd floor window and drags him to the street. He kicks him a few times, feels up his body, then kind of chokes him. He's confused, he's overwhelmed with desire but also hatred because Joja has rejected him and will never love him. He runs off because he can't bring himself to rape or murder Joja.

Since this is all happening at night (when he worked) I'm guessing he was fired/quit the night he was trying to cut the handcuffs off and he got into a fight with Fatima and Vigilio. (Vigilio and Fatima are having an affair, Fatima liked Sergio, he played with her emotions. At the same time Vigilio desired Sergio, they had sex, but in that scene we can see Vigilio being on Fatima's side)

So anyway, he saw the garbage truck, jumped on and stayed at the dump all night until the early hours. He tried being and animal ( like Lorde his beloved dog), ate garbage, catched a rabbit (the poor bunny btw) and drank from a dirty puddle, but he got sick from the water. he realized it was fruitless, he can't be a real animal. So he takes off the mask. He sleeps in processing factory at the dump all day, woke up, took a poop and then disappeared into the night. Probably living as some half man/half animal/ phantom that stalks the night. (Until he gets arrested, anyways).

That's my idea anyway.