Is there a 'bad guy' in the story 'A Study in Emerald'?

Yes, there is a 'bad guy' in the story, but surprisingly it is not the killer. Although Vernet and the Limping Doctor committed murder, they did so for good reasons, and killed a monster, not a person. The Old Ones are evil creatures from space who have conquered mankind and like to inflict pain on them. Vernet explains that the one they killed liked to torture humans until they were driven mad. The detective is just doing his job and trying to catch a killer, and the narrator is helping him. Neither of them seems like a bad person. The 'bad guys' in the story are actually the Old Ones.