The solution doesn't make any sense...

You are supposed to guess a 3 digit code.

The clues:

I am without sin, like a newborn. I have lost the count of deaths.
Your sin is my sacrement. You will never know how many I have murdered.
Mother, I herby confess my sins. The dogs I've poisoned outnumbers the chicken by five.
Father, I've sinned. The men I've murdered outnumbers the dogs by three.

+ A picture of a dog and a dogman and a chicken and the numbers 1, 3 and 5 written multiple times around the painting on the wall.

Ok... so let's assume C = murdered chicken, D for dogs and M for men.

First clue tells nothing...
Second clue implies that the 3 digit code you have to enter is the amount of things he has killed.
Third clue indicates that D = C + 5
Forth clue indicates that M = D + 3
The painting indicate that it should be one of the possible combinations with 1, 3 and 5 (which it's not).

We have NOTHING indicating the number for C except the numbers on the wall. Which gives us the following options:

135 -> 135 + 140 + 143 = 418 (not correct)
153 -> 153 + 158 + 161 = 472 (not correct)
315 -> 315 + 320 + 323 = 958 (not correct)

The remaining combinations 351, 513, 531 give totals over 999.

WTF... if we are just supposed to assume any possible C you get possible answers from C = 0 to C = 328 (highest before going over 999).

No what you are supposed to assume (apperently!) is that C = one of the numbers on the wall (not a combination of 2 or 3). You are supposed to assume it's 1 and that the sequence is NOT the total but the numbers of C, D, M entered in the order indicated by the picture: DMC

WHY???

Poorly designed puzzle by people who don't know maths very well I claim!

Oh, you poor thing. You massively over thought this one. Each number of the code consists of a single digit. Therefore if men is dogs plus three and dogs is chickens plus five and none of the number can be greater than nine because of the single digits then the numbers become quite clear, and the painting shows you the order. I don't know where you got the numbers 1, 3, and 5 from, I didn't notice them anywhere. It is actually not a difficult or poorly designed puzzle at all.

They are on the wall, clear as day.

Unless they redesigned it in a patch and fixed this puzzle. Just check the painting/wall, clear as day. How could you miss them (provided they didn't patch it)?