Why McMardo and McGinty thinks that Wilson is an informer?

McGinty, McMurdo, Baldwin, and the other men selected for the committee meet to discuss the plan. McMurdo identifies the man as Steve Wilson, lodging at Hobson’s Patch; Wilson was pretending to be a reporter. McMurdo then says his plan is to go to Wilson with his “secrets” of the Lodge for a price. This could happen at the trusty (and deaf) Widow MacNamara’s house. The seven men will meet there by nine; Edwards will show up, and they will take care of him.