Hm, sounds like in his realization of his wife being Dead, the reaction had come out as a person on the brink of extreme duress and disbelief. Thus the wavering of the vocals would, to a some, sound like laughter in a sense. A person on the verge of madness would as we've seen Rick, express this.
He's suffering from hysteria. He's trying to scream, sob, and talk all at the same time; he can't breathe. So the sobbs will come out choppy and irratic, making it sound like he's laughing.
Or hell, he's having a mental break down, there might have been a few laughs in there.
I thought it was pretty clear he was having a hysterical reaction to the news of his dead wife. Hysterical as in hysteria. As in his sanity went out the door. I dunno, I didn't think it sounded like laughing. It hadn't crossed my mind til someone else pointed it out. Too busy having my feels completely broken by the scene to even notice emotion_facepalm
He wasn't laughing really. I thought it was he just realized Carl did it and thought, "Oh my gosh, no way did my son just do that!" I just hope he doesn't hate Carl for it is all. He already avoids the baby as it is. sad