At this point, we really don't know much of anything, because the shooter wasn't white. The media can't even bring themselves to describe him as "hispanic." When George Zimmerman shot and killed Trayvon Martin, they labeled him "white hispanic." About the only news outlet that's actually dug a bit into his life is the U.K. Daily Mail. They are usually the ones to look to for what the main stream media refuses to cover. They're not perfect, but they are more motivated to dig for info than any U.S. news outlets.
“Do we even know for sure that the 19 kids were still alive at that point?”
There were 19 law enforcement officers assembled outside the hallway at 12:05. A few minutes later (I forget the exact time) a student in the room called 911 to report 8 or 9 students still alive. Shots could be heard in another call later at 12:21. The door was breached at 12:50.