I can imagine because not all shooting victims immediately die. A lot die slowly in agony and this sick SOB probably got off on it. I saw a video a few weeks ago of this black guy sitting in his car when someone walked up and shot him TWELVE times and incredibly the guy was still alive and conscious and started calling for help, he even had one bullet hole to his face, but he eventually died.
The sole survivor of the Babi Yar mass shootings survived by jumping down the mud slope ahead of the bullets, and being buried beneath the bodies of the dead. Later, wandering around filthy and naked, she found refuge, but only after being betrayed by a Ukrainian national who went out to inform on her, and warned to run for her life by a Wehrmacht soldier with a kernel of human decency in his breast. Nothing to do with your point, but an interesting anecdote.