Shane didn't pull the trigger, but that doesn't mean he WANTED to, he's a COP and he's not STUPID. If he had, what would have happened? Rick lays dead on the ground, shotgun wound to the back. Others come running at the sound of gun fire. They find Shane there and Rick dead, clearly shot in the back. That immediately puts him in a very bad spot with the rest of the group. Even worse, Dale was in the immediate vicinity, he would have likely seen Shane murder Rick. So he'd be forced to either kill Dale, or try to explain to the others what was going on. If he killed Dale, then he has to explain TWO deaths in immediate succession by his own hand with no clear reason.
Action, effect, fallout. You gotta think about these things. He didn't kill Rick because it was simply an act he couldn't get away with at that time and he KNEW it.
On the other hand, he killed Otis because he had reason to suspect he could get away with it. And with many of the group, he did. Dale having seen Shane about to murder Rick knows Shane well enough to suspect his story.
Please people, study cause and effect. It all then becomes so obvious.