During the 2001 election, I saw McCain in person a few times during the debates when all the Republican candidates came to our town.
Even our state senator and mayor noticed that McCain, when the camera is not on him, is mentally unstable. He has an anger that is barely controlled and I've seen that bared teeth look you described, demkicker.
His politics aside, I have always thought of him as a mental loose cannon. If elected president, he would be the US watered-down version of a NK Kim Jong. That is just my opinion and not meant in a mean-spirited way.