Carl was talking about how one action led to an unwanted result. He didn't kill te zombie in the swamp that killed Dale. He mentioned Rick didn't kill someone that killed mom. Did I hear that right? Did we know the walker that ate Lori?
Andrew was the prisoner that Rick chased out, who was presumed dead but returned the next episode and let all the walkers in. His actions indirectly led to Lori's death by separating the groups.