Timeline of the Shane/Otis thing (single line per event for a sense of timeflow):
Shane and Otis go to get supplies. As they are leaving, Otis puts his rifle, the same one he shot Carl with, in the vehicle. Shane glares at him. Otis says something along the lines of, “Sorry, it’s the only one I have. It’s the only gun here, actually. I need to take something with me.” Rick gives him his sidearm to take.
Rest of the gang is brought back from the highway and filled in on the situation.
Shit goes down with Shane and Otis.
Shane returns and tells the whole group they were out of ammo in the rifles and down to pistols. Also that he had 5 rounds and Otis had 4 (or vice versa). Dale seemed to notice that Shane had two pistols at that time, as well as both rifles.
At the memorial for Otis, Dale notices Rick has his sidearm again even though he’d loaned it to Otis and Shane and Otis supposedly had a single pistol each(something that Rick really should have noted, himself)
Dale, being more suspicious now, decides to get rid of the guns and takes them to the swamp. Shane follows and confronts him.
Dale says, “I know what kind of man you are. I saw you draw down on Rick. I think you shot Otis or left him there so you could escape.”
Shane laughs and says, “Give me the guns. You think I could shoot my best friend in the back? You think I shot someone I don’t even know? What do you think I’d do to someone I don’t like?” and grabs the guns and leaves.
Dale goes to Lori and tells her, “I think Shane shot Otis. He’s a loose cannon.”
Lori repeats this to Rick.
Rick confronts Shane and goes on a fishing expedition by saying, “I know what really happened.” without actually saying what happened.
Shane confesses to it, claiming to have done it to save Carl.