Some who view the video may conclude that the man must be insane, but I disagree. It proves his sanity by showing his clear rationalizations about why nothing he did at that point, and nothing he would do, was his fault. To do what he did he had to deliberately remove remorse and empathy for others from his emotional toolbox. He had been training for the day when someone would take an action where he could justify responding with deadly force.
That doesn't contradict the reported view that he had been assessed by someone, at least unofficially, as disturbed.
He had to recreate reality to fit his short-term needs. Likely the top need was the need to be absolutely in the right. He decided that the punishment for anyone reacting to him in an unfair way was death.
In a nutshell, his rationalization was, "They started it and they gave me no other choice."
Once someone starts buying into this way of thinking, every piece of information which contradicts this belief becomes suspect or is flat-out ignored. Most people who think this way never escalate to this level of violence, but a key difference here is the fantasy that the person is powerless in life to create a more positive life experience because all the people around him are intentionally making him powerless.
There is also a sense of megalomania in this video. Maybe in life he didn't feel in control or properly respected, but in death he will not only have control he will have created a legacy none of those around him can match.
What he failed to understand is that Professor Liviu Librescu who sacrificed his own life to save the lives of his students has earned a legacy a thousand times greater than the legacy that comes from murdering that professor and 31 other people. Each of those other victims also has a far greater legacy than he will ever have. They all faced frustrations in their lives and made multiple choices not to take those frustrations out on others at the point of a gun.
We can't let what this one man did overshadow the lives of those who died on Monday.