That is most definitely Assassin. I'm sure they used the SiN Sack kanji because of their facelessness making them have no identity, just like sin sacks.
On the pixiv, all the oni character names are tagged. I'm not saying what I expect is always what I get, but if he intended that to be assassin he would have tagged it. Seeing as it's also not tagged for sin sack on pixiv, I'll use neither here.