LE arriving on the scene is not going to shoot anyone. They will put you at gunpoint - especially if the "gun holder" puts his hands up - or even lays it on the ground. The second you see LE on scene lay it on the ground in front of you. LE will draw and order you to drop it - they aren't just going to shoot.
By re-holstering it - it was now out of sight and that could be more dangerous in that situation. What if he hadn't told LE about the gun and when LE was patting him down found it? Could be a very dangerous situation.
Put the gun on the ground between your feet - unless people are very close to you - no one is going to try to grab it and, in this situation, I don't think anyone else was going to draw a gun and fire at you.
Besides we're talking a matter of seconds here. gz hung up with 911 - 7:15; gf's call disconnected 7:16; LE arrived on scene 7:17.
JMHO
BBM
But he did. The first words out of his mouth was 'I shot him'. IMO I think he thought he was doing the right thing by putting the gun away (because that's what they train you to do, reholster). Keep in mind he may have been in shock that he did it instinctively too.