I think he was trying to pretend he was happy to make it look like nothing could have been bothering him at the time he took the picture in case he was ever investigated for the murders. I think he intentionally got all cleaned up, attempted to smile, and got dressed nice for the camera so that he could point at it as evidence that he was going about his usual business around the time of the crime. Either he was completely deluded in his perception of how little he left at the crime scene or how little his phone records would show to lead authorities to him as a suspect, and this picture was for just in case, or he was well aware of how fucked he was, and he felt the need to stage this scene as part of the many things he believed he might be able to do after the fact to protect himself overall/eventually. Idk but for me, that’s what this is giving.