90 percent certainty based on what? He was calling Murasaki back to no avail. Indeed it was his fault he did not stop her before it got worse, but that precedes the issue of him apologizing after the fact. Murasaki had already said very insulting things to the punks, so I doubt they would simply ignore her at that point. If he just took Murasaki and turned around, the punks will probably grab him back and fight with him. At least that is what I think.

Unfortunately, there is no sure way to tell how the punks would have reacted because it never happened. This being the case, we can't really be sure if Shinkurou did the right thing or not.

I do agree that Shinkurou is not a saint, or the typical admirable character, but neither is he a loser with no pride that you make him out to be. It is just that his values differ from the usual, but it was still an act with consideration to the situation. I personally believe that this gives depth to his character and makes him a bit more interesting, but this is a point of preference.