Were you surprised Bamber loaded a rifle and went outside to shoot rabbits ? He had not lived at WHF for a long time.
He had just worked a 14 hour day and you said Bamber was the laziest man alive.
I think that there was so much of a to-do going on that Jeremy just hung around before he said his goodbyes and to kill time ( pun ) he ventured outside with the rifle possibly trying to gain attention from his father that he was going to actually shoot something for once instead of being the wimp and hapless son that his father had thought he was. There's a difference between " target-practice " and killing something which was living.
Even though Jeremy had gone on the odd shoot,it wasn't known whether he'd ever joined in with the actual shooting.