I think you are on the right lines. Michael states A deer has to be taken with one shot but he still kills it in first hunt. Why one shot, to respect the deer or even so it doesn't even realise / have trauma around it's death (to offset Michael's guilt. He is forced to play Russian Roulette where this is not the case. This teaches him the value of life and he can't bring himself to kill the deer during the second hunt...