22.214.171.124 wrote: A letter in his house mentions a guard was supposed to stop by.
Maybe they were his remains.
I don't believe that. See, if he's doing the Sacrament I think it's precisely because he can't take care of a murder by himself (you know, as any random child) : too young, too afraid. I don't think he has it in him and at least in my language he pronounce the incantation with a frighten voice.
And if it was a guard, it wouldn't take long for the fellow soldiers to notice that disappearance. I doubt it was from the cemetery either. He'd have had to exhume the remains fairly swiftly since the piece of flesh and heart seem relatively fresh during the ritual.
Bones and heart and flesh don't necessary came from the same place. I can see him patiently assembling a skeleton at home piece by piece. I don't know if you need the full skeleton of a single person but in Windhelm you can either think of the cemetary or the Hall of the Dead. In that last place, you have bodies fresh and not so fresh so he can also steal the heart and flesh. Otherwise, maybe stores like Calixto's and the White Phial sell these ingredients. That would explain how the inhabitants seems to know what Aventus is doing.
If you think about it, Aventus was sent to Honorhall Orphanage. Why? Because his mother died. Obviously she wouldn't be left in the house, she'd be taken to the Hall of the Dead. So Aventus goes off to the orphanage and later escapes and comes home. Aventus quickly develops a hatred for Grelod so he would be home very quickly. Quickly enough perhaps because in "Blood on the Ice" we know that the woman in the Hall of the Dead has to prepare the body for the coffins and afterlife. So Aventus may have come home quickly enough to find his mothers body, rather than having to search through coffins of skeletons. His mothers body is the only one still fully intact and whole. So he gets the body home and is only just noticed by someone (that's how people know he's doing the Black Sacrament). The body, the bones and everything are his mothers. That way he didn't have to kill anyone, there's an already dead person for him to use. When you walk into his house, his voice is shaking and he is practically crying. Not because he's been doing the sacrament for a long time, but because he destroyed his dead mothers body to get revenge on Grelod. Remember he was doing this to kill her so almost nothing would be too out of the question for him to do.
It's a possibility though I don't believe it, we don't know when his mother died but surely she's not a skeleton yet (the flesh needs time to decompose) and preparing the body in Skyrim is actually doing rituals and maybe extract vital organs considering the tools they have around. Not removing flesh.
Another thing to consider, Aventus is around 10 so I don't see how he would manage to get a body home by himself wether it's his mother's or not.
As for how people know what he's doing and why he's crying, I think they hear him doing the Black Sacrament. To me, he's almost crying because he seems to be doing the ritual for days, he keep saying that he's tired, he's desperate and probably scared of the consequences of his success or failure. After all he's a kid asking for a favor to the wife of Sithis.