Obie is the one member of The Vigils who doubts what they are doing. He does not want to involve Jerry because he feels sorry for the fact that Jerry's mother recently died. Obie hates Archie and hates what he does to people, but he is not strong enough to speak out against Archie. Perhaps he is afraid, and simply wants to avoid the fallout that would result if he defied Archie or voiced his opinions against him. Obie could have blown the whistle on Archie at any time, but does not, perhaps because he feels it would be useless, or perhaps because he fears that he would then become Archie's next target.
Obie's one attempt to get back at Archie comes at the end, with the black box. The only reason Obie can get away with it is because all the students are watching, and Archie cannot back out. However, Archie handles it easily, and it ends up making Archie even stronger. Obie is outsmarted by Archie at every turn and knows it. That knowledge seals his involvement with The Vigils and Archie.