A man not leaving when told to leave is not a sufficient reason to intimidate him with a gun and then shoot him, especially when he was there to get his rightful legal custody of his son.
The smaller guy should have called the police and had them remove him. No one needed to die over this bunch of stupidity. The son didn't need his father killed.
Teal guy should have left the property when told to due so and called the police over the custody order. The gun owner was dealing with an obviously hostile and intemidating INTRUDER. Teal guy was clearly the agressor.
And you are sure things would not have escalated before Police had arrived. How many times have we seen the very same scenario and the boyfriend beaten or dead the wife beaten and hostage and the child a hostage as well?