In this situation, since the officer had no official business in the IRS building, his response should have been 'Ok I will leave." The Guard was the authority, right or wrong about the officer being allowed to carry his weapon inside. From the guards perspective a person was trying to enter with a weapon. He gave the officer the suggestion to put it in his cruiser. He could not do that while on duty. He said "I can't do that." then walked away. He said he was trying to de-escalate the situation by was leaving but did not say that to the security guard. So from the Guard's perspective he had rejected his suggestion to take the weapon to the car and had refused to dis-arm, then walked towards the elevator apparently tying to enter the building. Given those circumstances, and his belief that no on had the authority take a weapon into the building, his reaction was not as drastic as it first appears.
The officer said "I would say ‘Clearly your training is lacking and the fact that you went 0 to 100. Lethal force is unacceptable," said Gaston. Well, officer Gaston, how does it feel to be on the other end of the barrel? Cops are trained and expected to go from 1-100 and use lethal force. They are expected to "go in hard, and fast." They are trained to "take charge" and "secure the situation" to control the situation, preserve the scene, and provide for THEIR SAFETY and the publics. See how a cop going from 1-100 in three seconds worked out for Philando Castile.