Never pull (or show) a gun unless you are ready to use it.

However, there may be some justification for responding to an "unarmed" man with deadly force.  If the innocent victim of attack "reasonably fears that he/she is in danger of imminent great bodily injury or death and believes that a lessor degree of force is inadequate to deal with the situation".

"Reasonable fear" depends both on the ability of the attacker, and the ability of the defender.  If Mike Tyson (unarmed) comes up to you and threatens to "bust your skull", a reasonable person would expect to experience imminent great bodily injury.

If the scooter driver was elderly or handicapped his ability would possibly be reasonably less than the ability of this attacker.

This is the law as written in CRS 18-1-704(2).  
And, this is why the police are not charging the victim.

Now, all the DPO Lawyers will start arguing about how only police can get away with this, or other such nonsense concerning the use of deadly force.