He did not get past security, he attacked security. He shot the guy that was there to keep him out. Too bad his earlier convictions did not keep him behind bars so this would not happen.
I lived in DC when that museum opened. I worked at DIA, we got to tour it before it opened to the public. Then we took my wife's relatives there. It gives you cause to sit and ponder how man can be so cruel to fellow man.
I won't say this guy was sick, he was twisted.