I think the killing was rehearsed. Not necessarily for Alex specifically.

1] The shooter stares continuously at the disarmer placing his hand on, then removing Alex’s gun. This lasts for multiple seconds.

2] The shooter maintains arm-to-arm contact with the disarmer, a move commonly taught so you know where your partner is and what they’re doing.

3] The shooter pauses before drawing his own gun. When he draws it coincides the disarmer removing Alex’s gun

4] Upon disarm, the disarmer immediately turns around and runs. They don’t pause when the shooting starts

5] Upon disarm, the shooter immediately fills the space left by the disarmer.