Sometimes a symbol represents different things for different people. It's obvious that most Americans associate the National Anthem with what has made America a great country. Without waxing poetic, it's obvious to me that Kaepernick's protest has nothing to do with our military or the veterans who have fought [and lost their lives] is service of our country. To me, it's obvious that he is associating the National Anthem with the leadership and social/political policies that has failed a sector of our population. The reasons and responsibility for these failures is debateable, with vastly different perspectives, but really isn't relevant to this post.
From my observation of this situation, Kaepernick is quietly, and with respect for those around him, protesting a system he feels is broken. We all have the right and obligation to stand up (or kneel) against something we feel is unjust or wrong. I believe those who have expressed anger and venom are projecting their own feeling of what the National Anthem represents onto him. I would challenge us all to try to set aside what sitting through the National Anthem means to us individually, and try to look at his actions through his eyes. I don't agree that what he is doing proper, but neither do I believe he is attempting to disrespect our veterans.
My father was a Marine helicopter pilot in Viet Nam and I have a brother who just retired from the Air Force after 23 years service. I grew up singing the Marine Corps Hymn and many other patriotic songs as we took long trips. I was also taught to respect others' opinions and to always try to see things through another's eyes before judging them. We are all a collection of our individual experiences: not only are our experiences different, but the way those experiences have shaped us are different. Only by embracing those differences can we begin to understand each other.