What is more gruesome about Olson's murder than most of the previous ones? What reactions are there?

Olson is repeatedly shot because he keeps getting up. At one point, the boys near him see Olson trying to walk as his intestines spill out. Abraham throws up while Garraty begins to cry. Garraty wants to help Olson, but McVries pulls Garraty up because Olson is beyond help.