When you're in a high stress physical altercation with obscured vision, and you see a gun in your opponent's pants, and then a hand on that gun, and then the gun being pulled out of the pants by that hand, all happening quickly in succession, you may believe in that split second that you are in a mortal situation. These are not well trained soldiers here.

I'm not defending them, just guessing at the explanation. I'm operating from the assumption that the ICE agent did not randomly with absolutely no cause decide to execute a guy; probably something made him believe in that moment that he should shoot. You can see from the other, closer video that he very suddenly draws his weapon, as if in reaction to something.

Edit: someone else ITT theorized that the disarming agent ND'd which caused another agent to shoot in reaction. That's also pretty plausible.