Motive
As to why he chose that building or that time, we will never know.
This guy was psychotic. He rambled on in his recordings that he didn't want to do this, but he had to. For all we know, he had a class in that building and thought all the students were out to get him. Perhaps he thought aliens told him to commit the murders in that building. I don't think you will be able to pick up any patterns as I'm sure one person's insanity is vastly different than that of another. No doubt his insane mind felt wronged and the only answer in his deluded state was to shoot up this one buidling for what ever reason. Perhaps it is as simple as it was a building in the middle of campus that was furthest from police. Perhaps it had the fewest doors that were the easiest to chain shut. Perhaps he just knew the building because he took classes there. Perhaps is was random. Whatever reason - this was one sick dude. He didn't even know the first poor girl he killed in her dorm. No doubt with her he imagined some rebuff or took a liking to her and realized she would never like him.
Sadly, the motive is lost in his psychotic mind. He had no real reason to kill all the people, and as for why he chose that one building, it was either random, or the only positive that can come out of it is that the building made for an easy target and you then learn how to make buildings like that safer, BUT even then any lunatic with a gun won't be stopped. That then leads into the whole arm the faculty/students argument that the public is pretty much against.