Shane fights Morgan because he feels there is no other choice. In response for what Shane did to Chris, Morgan asserts that there must be justice done. For his part, Shane welcomes what he knew would be waiting for him. He understands that he did what he did to Chris because he had no other choice. In much the same way, Shane understands that he has to fight Morgan because there is no other way. Bob realizes this himself, seeing "the rightness of it."
Shane has to fight Morgan. There is no other way. Seeing that Morgan has come in revenge for Chris and that Shane's actions with Chris were forced, this confrontation is something from which Shane cannot escape. He understands it and accepts it. It is for this reason that Shane agrees to confront and fight Morgan and his group. It is also for this reason that Shane furthers his characterization as a complex figure. He has a strong moral code and must abide by it, even in situations that are not to his advantage such as his confrontation with Morgan. However, it is with this strict adherence in mind that he agress to fight Morgan.