Flame80, I believe the simple answer to your questions are that "Leaders lead by example" and the U.S. Rep. missed his opportunity.
I am not going to pretend to be an expert on the responsibilities of those in elective office. However, people in public service are, for the most part, held to a higher standard for conduct because they are perceived to be representatives for ALL the people of their jurisdiction (Municipal, State, Federal). As such, they serve our community/country best when they set the example for respect, tolerance, and unity (United States...).
Therefore, when someone in their presence makes even a casual threat to the life of a sitting leader, especially the President, and hyper-especially since AZ, the civic leader would serve us all well by immediately identifying such comments as unaccepatable ON THE SPOT. This would send a much more powerful message to the veracity and sincerity of his rebuke than "ignoring it" and later submitting a written statement of his actions/opinion after the fact.The proper response in my humble opinion: "Sir, your question is out of line and tantamount to treason. I will have to call for security to take you into custody until such time as the Secret Service can interview you. I will not tolerate any such remarks against our Country or any of its leaders. Let us continue with more appropriate questions."
That is the tone I'd like to see set by Americans.