Pretty clearly a hammerless Glock. It looks a bit largish in the murderer's hand, so there's a good chance it's a large-frame .45 Glock 21 or 10mm Glock 20.
The autopsies indicated fatal hits to the head and torso of both TV station victims, according to the New York Times. That sort of indicates he finished his indicated victims off with *certainty* headshots, but did not do so to interviewee Vicky Gardiner, who was injured badly enough.
The strangest thing about that video is how he walked up to them with the gun drawn and pointing at the reporter, who looked at him, and neither she nor the interview subject noticed anything unusual. That’s a strong argument for having a bit of situational awareness.
IMHO, the 10mm is a connoisseur or handgun hunter’s weapon and yet seemed more controlled (ie. less muzzle flip) than a rapid fire 45acp.
I agree the the pistol does look large in his hand.