Op came and leaked the answers to the youtube riddle. I figured there might be someone out there who wants an explanation for the rest of the puzzle. The updated info is at the bottom.
Who killed Nat Sow?
bleh, aka op
Answers to the riddles found in: i.stack.imgur.com/yCRns.png (linked by Deusovi).
One of five hundred
500 == D in roman numerals
Right below number one
== E, take the first letter (D) and go down one.
what sees, what is oneself
i (eye, me)
the gaping hole, the void
??? A letter that was missing because OP is dumb
whose uppercase version is lower than itself
dash - if you uppercase on a keyboard (SHIFT) it's underscore _
the number that is neither good or bad
the one that is 137 and 89, the twin of another
(89)16 = (137)10, and in the EBCDIC encoding schema, those are the codes for i (don't know what I was doing, meant to use ascii)
the latter of the middle brothers
m,n -> n
the one that is 9 after me, but 11 before
play on words of IX being 9 and XI being 11
and finally, the first part of the link
you-tube == u
And we get
https://www.youtube.com/watch?v=DEior-0inxU Vermeer? Wait, what's that painting on OP's profile pic called? (Girl with a Pearl Earring - Johannes Vermeer)
So now we take a look at the words imprinted into the man's skin:
kartisanship and vowzhwwpaeq
is an anagram for thisisaprank. This is our key for a Vigenère cipher. Plug in vowzhwwpaeq, and you get OVERPOWEREDF (probably not supposed to have an F).
These two clues combined:
OVERPOWERED has an abbreviation of OP, and Vermeer painted OP's picture, leading us to believe it is bleh who killed Nat Sow. As bleh stated before, the final answer was leaked in chat a while ago. I'm just updating this so those who don't check it can see.